This topic comes up in other threads sometimes, and I think it deserves its own thread.
I've kept a cold water (temperate) biotope tank for two years, with California two-spot octopus (O. Bimaculoides) and other compatible animals, all of which I've collected myself (legally), so I can help answer this question.
"First, do no harm"
1) Please, never release anything you catch back into the ocean if there is the slightest possibility that it might have been exposed to any non-native organism. Since it's extremely hard to avoid all store bought food, non-native tank mates, non-native refugium plants, live rock, etc, for 99.99999% of us this rule means "never release anything, ever", and also, "never give anything to a fish store, or to someone who might release it." If you do the math, you'll soon notice that this rule boils down to: "You catch it, you kill it". The point here is to keep non-native diseases, and parasites out of the native populations. This is extremely important, because local animals may have no defenses against exotic diseases or parasites, and some whole species could be devastated, all because you released your beloved "Mr. Fins" back into the ocean when you had to tear down your tank. If you are not the kind of stone-hearted SOB who will be able to flush "Mr. Fins" if he hasn't died of natural causes by the time you need to tear your tank down, then please don't collect local animals for your tank. You need to protect native species from exotic diseases - Mr. Fins will understand.
(I know of one loop hole in the "You catch it, You kill it" rule: If you live in a place, like the Florida Keys, where some local native animal species are already being sold in local fish stores, then it won't increase the risk to native populations if you give your animals, of those species species, to the local fish store, but you still shouldn't rlease them, or give new species to the LFS.)
2) Never catch anything and put it in your tank unless you have done your home work, and think you will be able to keep it alive and happy. Not only is it bad to mistreat animals, but when you see that an animal you caught isn't doing well in your tank, you are much more likely to be temped to break rule number one, and release it (and it's newly acquired exotic diseases) into the wild. If it's a one-way trip to your tank, only catch what you can keep alive.
a) Temperature: Can you match the natural water temp for that animal throughout the year? For example, the average monthly water temperature in Southern California, where the Catalina goby lives, ranges between about 57 in February and 70 in August. If your chiller can keep your tank at 70 all year - that's not cold enough, the fish will be stressed and die early. In nature, it only gets up to 70 for a couple of weeks per year, so you either need to match the natural ups and downs, or keep a steady 63-65 degrees all year. If you can't afford to buy and run a chiller that can match the natural temps where you catch your animals, then please don't catch any.
b) Food: don't catch thing that you aren't likely to be able to feed. Nudibranchs, filter feeders, etc can be very hard to feed.
c) Everything else: Tank mate compatibility, tank size, light levels, etc. Do tons of research before you decide what to catch for your tank.
Laws:
Look up, study, and follow all the laws concerning taking marine animals where you live. For the most part, the laws are good-sense measures to protect natural resources. They can be complicated, easily misunderstood, and costly if you get fined, so learn the rules and play by them - It's your butt if you get the rules wrong. You'll probably need a fishing license, which comes with a regulation book that summarizes the laws. A few places have special rules (game reserves, protected areas, etc) and those places will be carefully defined in the regulation booklet. Just because a place is, or is not, called a "state park", or whatever, it doesn't mean that everything in it (or anything in the ocean) is necessarily protected. Don't make assumptions, read the booklet carefully and nail down exactly what the rules are and where the boundaries of the truly protected areas are. Also, be aware of open and closed seasons, size limits, equipment restrictions, etc. Remember, laws only tell you what is NOT legal. Everything else IS legal, even if the regulation book doesn't mention it at all. (but watch out for catch-all terms like "all fin-fish" or "invertebrates")
Collecting your own animals:
I have a temperate tank with local animals from Southern California, and I collect all my own stuff. I agree with SJreefer; get lots of diving experience before you collect animals. Collecting animals while scuba diving causes you to focus all your attention on what you're doing with your hands, and this can make you unaware of what's going on around you, especially if you are not an experienced diver. In such a situation it is easy to not realize, for example, that your buoyancy is off, and that you are sinking, too deep, or that you are in an uncontrolled ascent, either of which can be deadly. You want safe diving practices to be second nature to you before you allow yourself to be distracted so much. On the other hand, you can dive and collect in rather safe locations, like in 25 feet of calm water inside the rock jetty of a harbor, where there are far fewer ways to get into trouble, but still lots of neat things to find for your tank.
In California, some things are legal to take from tide pools at low tide with a fishing license (octopus, crabs, clams, urchins, snails, etc.) so if that's true in Florida, then you don't need to be able to scuba dive for some things. Check out an online tide table for your area and see when the super low minus tides will be (mostly November through February) and go turn over rocks at low tide to find things (always flip the rocks back the way they were so the encrusting animals won't die). You can catch fish in tide pools too ,with nets, or hook and line, just study the fish and game regulations so you can avoid a big ticket.
In shallow water, it's also possible to freedive for animals (hold your breath and dive with mask, fins, and snorkel) but that also takes time before you're good enough to stay down long enough to grab animals. If you go that way, study up on "shallow water blackout" and learn how to avoid it - and never stay down long enough to push the limits - keep it comfortable, and don't die. I love freediving, but I have to admit that SCUBA is a much easier way to collect for your aquarium than freediving is.
Here are a few general tips I can offer:
1) Study and learn about the animals that live where you'll be diving. Learn what is dangerous, what is not, and how to positively ID everything.
2) Make a rule for yourself: If you're not sure exactly what something is, assume it can hurt you and don't touch it (that policy probably saved the tip of my finger from being sliced off before I knew what mantis shrimp were).
3) Find experienced people in your area and learn from their experience, and go diving with them if you can. Some things that seem mostly harmless can be a big problem, and the grizzled salty old divers in your area can teach you about them. In California, for example, sea urchins in surgey conditions can really mess you up. I know guys that got poked with urchin spines, that broke off, and worked their way deeper and deeper into their hand, foot, knee, or whatever over several months. If a spine gets into a joint, you're really screwed. Find a mentor if you can, and read books about local critters and conditions.
4) Don't try to net fish directly, they are faster than you. Instead, place the net in the fish's likely escape route, and use a stick, or your other hand, to chase the fish into your net (you are smarter than them!)
5) You'll often collect in rocky places, where there are lots of things on the rocks that can scratch or sting you, and lots of things that you can damage, so always wear dive gloves, and a wetsuit, or at least a dive skin. Also, work to develop these two habits:
a) Always inspect the spot where you are about to place your hand, to make sure that it will be safe for you, and not damage any fragile animal. Lots of animals use camouflage to keep safe, and will pretend to be part of the rock or sand until the last second, or even longer. Some of these hard to see critters have venomous spikes on them too (sculpin, stone fish, sting rays, etc), so always look before you touch a "rock".
b) Never let any part of your body, except your gloved hands, touch anything. This is impossible in practice, but it's a great thing to strive for, and if you habitually do both a) and b) as much as possible, you'll save yourself a lot of trouble.
6) In surgey conditions, hold your hands up in the "I surrender" position on either side of your head, so that as you are pushed and pulled, you can use your hands to keep your head and body from being banged into anything.
7) Never collect anything that you haven't already prepared to care for (at least mostly). Know for sure what it is, and with 90% certainty, what it eats, what eats it, and weather conditions in your tank are suitable for it. Ideally, do all your homework first, and have a short shopping list in mind of what you are looking for before you go diving. Some experimentation is inevitable, because there is very little information about the husbandry of animals except those in the pet trade, but try hard to minimize your experimentation. Some animals (some sea cucumber, octopus) can kill your whole tank.
8) Know the laws about collecting marine animals in your state, and if possible, study the text of the laws yourself. I've found that California Dept of Fish and Game employees often say that keeping certain animals (or any animals) is illegal, when it is in fact not illegal. It's hard to blame them; a game warden knows the lobster rules by heart, but you might be the first guy they've ever seen with a gorgonian in your bucket. I think they often just don't know how the laws apply to the odd animals we might take, and so they play it safe, when asked, by saying "it's illegal" rather than "Golly, search me, I'm stumped!" When I collect in tide pools, I carry extra copies of the applicable statues in which I've highlighted the sentences that make it clear that what I'm doing, and where I'm doing it, is not illegal. I can show them to hostile and misinformed eco-zealots at the beach who "know" that I'm breaking the law and demand that I stop, and I like being prepared in case somebody with authority considers writing me a ticket.
9) Be smart about transporting your animals from the ocean to your tank. If the trip will take more than an hour, you need to find ways to maintain the water temperature and oxygen level so animals won't be stressed before you get them home. Have ways to keep things separate so they can't hurt each other, and so that rocks can't slam into animals during the drive. Use insulated boxes, ice chests, battery powered air pumps etc. and the more water you have on the drive home, the better. I keep medium to large sized clear plastic bottles with screw on lids (holes drilled in the lids) (peanut butter, mixed nuts, etc) and carry several of them in my game bag when I dive, so that I can keep one animal in each jar. That way they are protected from each other, and won't be squished when I drag the game bag around.
I've kept a cold water (temperate) biotope tank for two years, with California two-spot octopus (O. Bimaculoides) and other compatible animals, all of which I've collected myself (legally), so I can help answer this question.
"First, do no harm"
1) Please, never release anything you catch back into the ocean if there is the slightest possibility that it might have been exposed to any non-native organism. Since it's extremely hard to avoid all store bought food, non-native tank mates, non-native refugium plants, live rock, etc, for 99.99999% of us this rule means "never release anything, ever", and also, "never give anything to a fish store, or to someone who might release it." If you do the math, you'll soon notice that this rule boils down to: "You catch it, you kill it". The point here is to keep non-native diseases, and parasites out of the native populations. This is extremely important, because local animals may have no defenses against exotic diseases or parasites, and some whole species could be devastated, all because you released your beloved "Mr. Fins" back into the ocean when you had to tear down your tank. If you are not the kind of stone-hearted SOB who will be able to flush "Mr. Fins" if he hasn't died of natural causes by the time you need to tear your tank down, then please don't collect local animals for your tank. You need to protect native species from exotic diseases - Mr. Fins will understand.
(I know of one loop hole in the "You catch it, You kill it" rule: If you live in a place, like the Florida Keys, where some local native animal species are already being sold in local fish stores, then it won't increase the risk to native populations if you give your animals, of those species species, to the local fish store, but you still shouldn't rlease them, or give new species to the LFS.)
2) Never catch anything and put it in your tank unless you have done your home work, and think you will be able to keep it alive and happy. Not only is it bad to mistreat animals, but when you see that an animal you caught isn't doing well in your tank, you are much more likely to be temped to break rule number one, and release it (and it's newly acquired exotic diseases) into the wild. If it's a one-way trip to your tank, only catch what you can keep alive.
a) Temperature: Can you match the natural water temp for that animal throughout the year? For example, the average monthly water temperature in Southern California, where the Catalina goby lives, ranges between about 57 in February and 70 in August. If your chiller can keep your tank at 70 all year - that's not cold enough, the fish will be stressed and die early. In nature, it only gets up to 70 for a couple of weeks per year, so you either need to match the natural ups and downs, or keep a steady 63-65 degrees all year. If you can't afford to buy and run a chiller that can match the natural temps where you catch your animals, then please don't catch any.
b) Food: don't catch thing that you aren't likely to be able to feed. Nudibranchs, filter feeders, etc can be very hard to feed.
c) Everything else: Tank mate compatibility, tank size, light levels, etc. Do tons of research before you decide what to catch for your tank.
Laws:
Look up, study, and follow all the laws concerning taking marine animals where you live. For the most part, the laws are good-sense measures to protect natural resources. They can be complicated, easily misunderstood, and costly if you get fined, so learn the rules and play by them - It's your butt if you get the rules wrong. You'll probably need a fishing license, which comes with a regulation book that summarizes the laws. A few places have special rules (game reserves, protected areas, etc) and those places will be carefully defined in the regulation booklet. Just because a place is, or is not, called a "state park", or whatever, it doesn't mean that everything in it (or anything in the ocean) is necessarily protected. Don't make assumptions, read the booklet carefully and nail down exactly what the rules are and where the boundaries of the truly protected areas are. Also, be aware of open and closed seasons, size limits, equipment restrictions, etc. Remember, laws only tell you what is NOT legal. Everything else IS legal, even if the regulation book doesn't mention it at all. (but watch out for catch-all terms like "all fin-fish" or "invertebrates")
Collecting your own animals:
I have a temperate tank with local animals from Southern California, and I collect all my own stuff. I agree with SJreefer; get lots of diving experience before you collect animals. Collecting animals while scuba diving causes you to focus all your attention on what you're doing with your hands, and this can make you unaware of what's going on around you, especially if you are not an experienced diver. In such a situation it is easy to not realize, for example, that your buoyancy is off, and that you are sinking, too deep, or that you are in an uncontrolled ascent, either of which can be deadly. You want safe diving practices to be second nature to you before you allow yourself to be distracted so much. On the other hand, you can dive and collect in rather safe locations, like in 25 feet of calm water inside the rock jetty of a harbor, where there are far fewer ways to get into trouble, but still lots of neat things to find for your tank.
In California, some things are legal to take from tide pools at low tide with a fishing license (octopus, crabs, clams, urchins, snails, etc.) so if that's true in Florida, then you don't need to be able to scuba dive for some things. Check out an online tide table for your area and see when the super low minus tides will be (mostly November through February) and go turn over rocks at low tide to find things (always flip the rocks back the way they were so the encrusting animals won't die). You can catch fish in tide pools too ,with nets, or hook and line, just study the fish and game regulations so you can avoid a big ticket.
In shallow water, it's also possible to freedive for animals (hold your breath and dive with mask, fins, and snorkel) but that also takes time before you're good enough to stay down long enough to grab animals. If you go that way, study up on "shallow water blackout" and learn how to avoid it - and never stay down long enough to push the limits - keep it comfortable, and don't die. I love freediving, but I have to admit that SCUBA is a much easier way to collect for your aquarium than freediving is.
Here are a few general tips I can offer:
1) Study and learn about the animals that live where you'll be diving. Learn what is dangerous, what is not, and how to positively ID everything.
2) Make a rule for yourself: If you're not sure exactly what something is, assume it can hurt you and don't touch it (that policy probably saved the tip of my finger from being sliced off before I knew what mantis shrimp were).
3) Find experienced people in your area and learn from their experience, and go diving with them if you can. Some things that seem mostly harmless can be a big problem, and the grizzled salty old divers in your area can teach you about them. In California, for example, sea urchins in surgey conditions can really mess you up. I know guys that got poked with urchin spines, that broke off, and worked their way deeper and deeper into their hand, foot, knee, or whatever over several months. If a spine gets into a joint, you're really screwed. Find a mentor if you can, and read books about local critters and conditions.
4) Don't try to net fish directly, they are faster than you. Instead, place the net in the fish's likely escape route, and use a stick, or your other hand, to chase the fish into your net (you are smarter than them!)
5) You'll often collect in rocky places, where there are lots of things on the rocks that can scratch or sting you, and lots of things that you can damage, so always wear dive gloves, and a wetsuit, or at least a dive skin. Also, work to develop these two habits:
a) Always inspect the spot where you are about to place your hand, to make sure that it will be safe for you, and not damage any fragile animal. Lots of animals use camouflage to keep safe, and will pretend to be part of the rock or sand until the last second, or even longer. Some of these hard to see critters have venomous spikes on them too (sculpin, stone fish, sting rays, etc), so always look before you touch a "rock".
b) Never let any part of your body, except your gloved hands, touch anything. This is impossible in practice, but it's a great thing to strive for, and if you habitually do both a) and b) as much as possible, you'll save yourself a lot of trouble.
6) In surgey conditions, hold your hands up in the "I surrender" position on either side of your head, so that as you are pushed and pulled, you can use your hands to keep your head and body from being banged into anything.
7) Never collect anything that you haven't already prepared to care for (at least mostly). Know for sure what it is, and with 90% certainty, what it eats, what eats it, and weather conditions in your tank are suitable for it. Ideally, do all your homework first, and have a short shopping list in mind of what you are looking for before you go diving. Some experimentation is inevitable, because there is very little information about the husbandry of animals except those in the pet trade, but try hard to minimize your experimentation. Some animals (some sea cucumber, octopus) can kill your whole tank.
8) Know the laws about collecting marine animals in your state, and if possible, study the text of the laws yourself. I've found that California Dept of Fish and Game employees often say that keeping certain animals (or any animals) is illegal, when it is in fact not illegal. It's hard to blame them; a game warden knows the lobster rules by heart, but you might be the first guy they've ever seen with a gorgonian in your bucket. I think they often just don't know how the laws apply to the odd animals we might take, and so they play it safe, when asked, by saying "it's illegal" rather than "Golly, search me, I'm stumped!" When I collect in tide pools, I carry extra copies of the applicable statues in which I've highlighted the sentences that make it clear that what I'm doing, and where I'm doing it, is not illegal. I can show them to hostile and misinformed eco-zealots at the beach who "know" that I'm breaking the law and demand that I stop, and I like being prepared in case somebody with authority considers writing me a ticket.
9) Be smart about transporting your animals from the ocean to your tank. If the trip will take more than an hour, you need to find ways to maintain the water temperature and oxygen level so animals won't be stressed before you get them home. Have ways to keep things separate so they can't hurt each other, and so that rocks can't slam into animals during the drive. Use insulated boxes, ice chests, battery powered air pumps etc. and the more water you have on the drive home, the better. I keep medium to large sized clear plastic bottles with screw on lids (holes drilled in the lids) (peanut butter, mixed nuts, etc) and carry several of them in my game bag when I dive, so that I can keep one animal in each jar. That way they are protected from each other, and won't be squished when I drag the game bag around.
