First of all, get rid of the algae. Corals don't like phosphate, (fish don't either)--and high phosphate will be a killer.
Removing phosphate: use GFO, do water changes, and if you also have nitrate, NoPoX can be a help. You want a level of phosphate that might sustain a little film on the glass now and again, but your rocks should be bare as eggs.
Second, get rid of the nitrate. If you have a weak skimmer, and it is not pulling much skimmate, but you have high nitrate---this is a problem. NoPoX can hype your skimmer in this situation, but if you have high nitrate (your goal is 5) and don't have dark skimmate, your skimmer may not be good enough. I fought this battle long and hard (and with a great deal of expense) until a potent skimmer showed me what a good skimmer looked like---the nitrate problem went down in DAYS. My recommendation for a larger tank and a serious problem is a cone-type skimmer that produces thick foam, not just bubbles. It should look like the topping on a cream pie. Meringue.
Now, if you have a small tank, water changes are easier for you. And a program of water changes being SURE your ro/di filter is on the job---helps. You don't want to overdo it so far that your tank could no longer be considered cycled, but do a little real often, and test every couple of days to see if you're making progress. NoPox DEPENDS on a potent skimmer, and might make matters worse without one. Rely on the water changes instead, and test the ro/di to be SURE you're not continually contributing to the problem.
Once you have your phosphate and nitrate hammered down to barely detectable, the next problem is LIGHTS. If you have T5's or reef-capable LEDs you're probably good for anything from softies up to lps stony (the fluffy stony coral, not the colored sticks (sps)) Buy ONE test coral, small, battered, and cheap, and see if you can make it happy. It should extend (puff up) and grow. Don't agonize over feeding it. Bristleworms break down food nicely to coral-feeding size, if fish-poo isn't doing it---and corals likewise 'eat' light: their 'suntans' ---that color---is their internal passengers (zooxanthellae---Greek for 'animacules of a yellow-brown color really little ones') process light into sugars, just as plants do. So if you would not offer hamburger to an African Violet, don't stress out too much about waving food past a coral...some have obvious mouths and will eat it (a plate coral can take down a thin slice of shrimp) but many just wave tentacles in the current and sop up not only food particles but calcium and such dissolved in the water.
Read the list of elements on your salt mix.
You also need: tests for salinity (1.024 to 1.026); for alkalinity (8.3 is good); and if your coral has a skeleton, for magnesium (1350) and calcium (420). The numbers I give you are middle of the range, so that evaporation or a little over-watering won't throw them badly. Try to stay mid range. Your temperature should be a pretty steady 78-79. And your lights should be on about 12 hours.
Corals aren't hard, but set up properly before you go there. Once you do get there, they WILL grow for you, and a lot faster than many expect.
Removing phosphate: use GFO, do water changes, and if you also have nitrate, NoPoX can be a help. You want a level of phosphate that might sustain a little film on the glass now and again, but your rocks should be bare as eggs.
Second, get rid of the nitrate. If you have a weak skimmer, and it is not pulling much skimmate, but you have high nitrate---this is a problem. NoPoX can hype your skimmer in this situation, but if you have high nitrate (your goal is 5) and don't have dark skimmate, your skimmer may not be good enough. I fought this battle long and hard (and with a great deal of expense) until a potent skimmer showed me what a good skimmer looked like---the nitrate problem went down in DAYS. My recommendation for a larger tank and a serious problem is a cone-type skimmer that produces thick foam, not just bubbles. It should look like the topping on a cream pie. Meringue.
Now, if you have a small tank, water changes are easier for you. And a program of water changes being SURE your ro/di filter is on the job---helps. You don't want to overdo it so far that your tank could no longer be considered cycled, but do a little real often, and test every couple of days to see if you're making progress. NoPox DEPENDS on a potent skimmer, and might make matters worse without one. Rely on the water changes instead, and test the ro/di to be SURE you're not continually contributing to the problem.
Once you have your phosphate and nitrate hammered down to barely detectable, the next problem is LIGHTS. If you have T5's or reef-capable LEDs you're probably good for anything from softies up to lps stony (the fluffy stony coral, not the colored sticks (sps)) Buy ONE test coral, small, battered, and cheap, and see if you can make it happy. It should extend (puff up) and grow. Don't agonize over feeding it. Bristleworms break down food nicely to coral-feeding size, if fish-poo isn't doing it---and corals likewise 'eat' light: their 'suntans' ---that color---is their internal passengers (zooxanthellae---Greek for 'animacules of a yellow-brown color really little ones') process light into sugars, just as plants do. So if you would not offer hamburger to an African Violet, don't stress out too much about waving food past a coral...some have obvious mouths and will eat it (a plate coral can take down a thin slice of shrimp) but many just wave tentacles in the current and sop up not only food particles but calcium and such dissolved in the water.
Read the list of elements on your salt mix.
You also need: tests for salinity (1.024 to 1.026); for alkalinity (8.3 is good); and if your coral has a skeleton, for magnesium (1350) and calcium (420). The numbers I give you are middle of the range, so that evaporation or a little over-watering won't throw them badly. Try to stay mid range. Your temperature should be a pretty steady 78-79. And your lights should be on about 12 hours.
Corals aren't hard, but set up properly before you go there. Once you do get there, they WILL grow for you, and a lot faster than many expect.