This comes up now and again---somebody trying to buy a used tank.
Acrylic or glass---both kinds of tanks have their cautions.
Acrylic scratches easily, grain of sand getting stuck under a magnet scraper---glass not so much.
Acrylic unsupported by bracing can, over years, bow outward with the heat and weight of the water and sand. You can deal with this, generally, but do be sure that the acrylic is thick enough and sturdy enough to hold up the weight of water. I have also had, over a period of years, 'crazing' of the acrylic, which are micro-cracks in the material, in an area where there was bowing.
Acrylic does bend nicely, and can eliminate some glued seams.
Both glass and acrylic are sometimes used for reptile tanks, and it's one thing to use thin material when its holding nothing back but ten pounds of warm sand and a small gecko. It's quite another when we add eighty degree salt water and load it up with 50 pounds of sand and another fifty of rock with pointy bits.
As a rule of thumb, the real, real thin glass is not going to be good for a marine tank, because of the weight-hazard. My 55 gallon and my 105 gallon both have 5/8ths inch thick glass, and are real heavy---took 3 people to lift the 105 onto the stand empty. Acrylic has the bennie of being light, is strong at a thinner (but not too thin!) measurement, and if you see a house move in your future, or enjoy the third-floor apartment life, that may be a good choice for you.
If you do see a lot of scratching, THAT will not be such a good tank for you: algae loves to grow in scratches. Still---it might be a good deal if it's otherwise sturdy, thick-ish, and a size you otherwise couldn't afford but need. Excepting the algae growing in the scrapes, once water is in it, the scratches won't be evident unless they're quite bad.
White haze or spots or even encrustation on the other hand is not a big deal: white vinegar will dissolve that overnight, and is perfectly safe to use around a marine tank.
DO ask whether copper was ever used in the tank. If it was, run it for a while with warm salt water in it (no sand or rock) with a product called PolyFilter ---which absorbs copper. It turns blue when it does. And once a new sheet of it does not turn blue, the copper is no longer being released into the water and the tank can be used. Polyfilter is not cheap, so the tank should not be too pricey.
A 'drilled' tank is a good find in the used market, if it is professionally (or at least well-) drilled and has a standard downflow and piping for a sump.
Your best choice if somebody wants to sell you a functioning system is to trade the fish off to a fish store, toss the sand, and keep the rock in a well-circulated dark saltwater bucket or tub until you can render the tank ready to use. If you like the fish, you do need to have them in qt during this process. Rarely is a tank somebody plans to sell in such tiptop condition that it is capable of being set up again and functioning...at very least those fish will need to be in a 5-day or longer quarantine, new sand has to be washed (doing that for a 100 gallon tank is an all day operation), the rockwork has to be set up, and in short---you're going to put sweat equity into that bargain tank. But they can be a really, really good deal. Just don't get starry-eyed expectations that that transfer to your house is going to be instant heaven, or the notion that that scruffy crusted tank is going to shine by tomorrow.
Acrylic or glass---both kinds of tanks have their cautions.
Acrylic scratches easily, grain of sand getting stuck under a magnet scraper---glass not so much.
Acrylic unsupported by bracing can, over years, bow outward with the heat and weight of the water and sand. You can deal with this, generally, but do be sure that the acrylic is thick enough and sturdy enough to hold up the weight of water. I have also had, over a period of years, 'crazing' of the acrylic, which are micro-cracks in the material, in an area where there was bowing.
Acrylic does bend nicely, and can eliminate some glued seams.
Both glass and acrylic are sometimes used for reptile tanks, and it's one thing to use thin material when its holding nothing back but ten pounds of warm sand and a small gecko. It's quite another when we add eighty degree salt water and load it up with 50 pounds of sand and another fifty of rock with pointy bits.
As a rule of thumb, the real, real thin glass is not going to be good for a marine tank, because of the weight-hazard. My 55 gallon and my 105 gallon both have 5/8ths inch thick glass, and are real heavy---took 3 people to lift the 105 onto the stand empty. Acrylic has the bennie of being light, is strong at a thinner (but not too thin!) measurement, and if you see a house move in your future, or enjoy the third-floor apartment life, that may be a good choice for you.
If you do see a lot of scratching, THAT will not be such a good tank for you: algae loves to grow in scratches. Still---it might be a good deal if it's otherwise sturdy, thick-ish, and a size you otherwise couldn't afford but need. Excepting the algae growing in the scrapes, once water is in it, the scratches won't be evident unless they're quite bad.
White haze or spots or even encrustation on the other hand is not a big deal: white vinegar will dissolve that overnight, and is perfectly safe to use around a marine tank.
DO ask whether copper was ever used in the tank. If it was, run it for a while with warm salt water in it (no sand or rock) with a product called PolyFilter ---which absorbs copper. It turns blue when it does. And once a new sheet of it does not turn blue, the copper is no longer being released into the water and the tank can be used. Polyfilter is not cheap, so the tank should not be too pricey.
A 'drilled' tank is a good find in the used market, if it is professionally (or at least well-) drilled and has a standard downflow and piping for a sump.
Your best choice if somebody wants to sell you a functioning system is to trade the fish off to a fish store, toss the sand, and keep the rock in a well-circulated dark saltwater bucket or tub until you can render the tank ready to use. If you like the fish, you do need to have them in qt during this process. Rarely is a tank somebody plans to sell in such tiptop condition that it is capable of being set up again and functioning...at very least those fish will need to be in a 5-day or longer quarantine, new sand has to be washed (doing that for a 100 gallon tank is an all day operation), the rockwork has to be set up, and in short---you're going to put sweat equity into that bargain tank. But they can be a really, really good deal. Just don't get starry-eyed expectations that that transfer to your house is going to be instant heaven, or the notion that that scruffy crusted tank is going to shine by tomorrow.
:facepalm: LMAO 
