the first step to fixing it is getting your own test kits, doing your own tests and knowing what the numbers are. Your LFS
is predisposed to say your water is fine playing the odds that you'll start looking at the fish while they have you in the store. Not all distort the truth but..... You need to know what the numbers are for (at a minimum) ammonia, nitrite, nitrate, SG
and PH. Also calcium, alk
, phosphates amongst others.
Pretty good rule of thumb: If it's green it algae. If it's algae it's either your lighting, you are letting too much sunlight hit the tank, your nitrate and/phosphate are high (or a combination). Remember, your nitrates and phosphates may test low because your algae bloom is eating them. So your tank produces more and you get more fat algae. And the cycle continues.
You also need to understand what you water params are before and after you mix the salt or if buying premixed then what they are before they go in the tank to know if you are adding nitrates/etc with water changes.
You could cover the tank and kill it off but if your water is the cause then as soon as the light comes back.,.BANG.. algae.
Also, lights lose their effectiveness and can cause an algae outbreak as well.
Just some of the things that need to be kept on top of.
O the joy eh?