In a word---phosphate. And please pass this on.
Corals don't like it. Fish aren't fond of it. And in the presence of phosphate, hair algae takes off in huge sheets and nastiness. Film algae forms on your glass. Unwanted macro takes off and chokes things.
So where's it coming from?
Number one source: water. Tapwater is treated only to remove what makes it unfit drinking water. Phosphate tolerances for US are quite high. Got the picture? Your tapwater may or may not be loaded with it and other things we tolerate.
Number two source: sadly, live rock. Live rock may bring in phosphate. You have to have it. There you are.
Number three source: bought algae---as fishfood. Even dry red flake or pellet fishfood has it. If you are feeding a herbivore, you are importing stuff that has phosphate in it. Kind of ironic: you get a tang or lawnmower to eat the algae you don't like, and when it runs out, you add more algae to feed the tang or lawnmower. Unfortunately corals really don't like phosphate. This is a problem.
And never mind buying a phosphate test: it will almost always show negative if you have a lot of algae. Why? It can't 'read' phosphate that's bound up in the algae. So save your money: if you have algae in your display, you have phosphate. When it gets loose from the algae, it harms your corals. It's a vicious cycle.
Now that I've delivered depressing news---how do you fix it?
There are chemical absorbents, an iron preparation called variously Phosban, Phosguard, etc. These are ok. But my favorite, and the one that works best, is setting up a meaningful fuge.
Lit 24/7, its algae, usually cheatomorpha, sucks up the loose phosphate that decays out of the main tank and makes it harder and harder for algae in the main tank to make a decent living. Eventually it gets discouraged out of existence. If you do have an algae-eating fish, it grabs the phosphate released by that fish's poo and does not let all of it get back into your display tank.
Once you understand the transaction, you see how it works: the fuge 'binds' the algae up, you keep cutting your cheato ball in half and selling half of it off, or just tossing it, and your water stays phosphate-free.
A fuge CAN'T outcompete tapwater. You have to improve that. But if you have an algae-eating fish, get a fuge going, and you should be in great shape to maintain healthy water, feed your fish, and even raise some lowlight soft corals like mushrooms (saying you have low lighting). If you have high level lighting, no reason you can't get such corals to thrive right alongside your fish. You'll have better than average water and one of the biggest reasons corals don't thrive in some tanks will just be cleared up to a no-problem.
Long post, but HTH, friends, so you understand why we keep saying "refugium."
Corals don't like it. Fish aren't fond of it. And in the presence of phosphate, hair algae takes off in huge sheets and nastiness. Film algae forms on your glass. Unwanted macro takes off and chokes things.
So where's it coming from?
Number one source: water. Tapwater is treated only to remove what makes it unfit drinking water. Phosphate tolerances for US are quite high. Got the picture? Your tapwater may or may not be loaded with it and other things we tolerate.
Number two source: sadly, live rock. Live rock may bring in phosphate. You have to have it. There you are.
Number three source: bought algae---as fishfood. Even dry red flake or pellet fishfood has it. If you are feeding a herbivore, you are importing stuff that has phosphate in it. Kind of ironic: you get a tang or lawnmower to eat the algae you don't like, and when it runs out, you add more algae to feed the tang or lawnmower. Unfortunately corals really don't like phosphate. This is a problem.
And never mind buying a phosphate test: it will almost always show negative if you have a lot of algae. Why? It can't 'read' phosphate that's bound up in the algae. So save your money: if you have algae in your display, you have phosphate. When it gets loose from the algae, it harms your corals. It's a vicious cycle.
Now that I've delivered depressing news---how do you fix it?
There are chemical absorbents, an iron preparation called variously Phosban, Phosguard, etc. These are ok. But my favorite, and the one that works best, is setting up a meaningful fuge.
Lit 24/7, its algae, usually cheatomorpha, sucks up the loose phosphate that decays out of the main tank and makes it harder and harder for algae in the main tank to make a decent living. Eventually it gets discouraged out of existence. If you do have an algae-eating fish, it grabs the phosphate released by that fish's poo and does not let all of it get back into your display tank.
Once you understand the transaction, you see how it works: the fuge 'binds' the algae up, you keep cutting your cheato ball in half and selling half of it off, or just tossing it, and your water stays phosphate-free.
A fuge CAN'T outcompete tapwater. You have to improve that. But if you have an algae-eating fish, get a fuge going, and you should be in great shape to maintain healthy water, feed your fish, and even raise some lowlight soft corals like mushrooms (saying you have low lighting). If you have high level lighting, no reason you can't get such corals to thrive right alongside your fish. You'll have better than average water and one of the biggest reasons corals don't thrive in some tanks will just be cleared up to a no-problem.
Long post, but HTH, friends, so you understand why we keep saying "refugium."