I had the VERY same issue, nothing would work, i'd cover my tank, stop feedings, constantly change water. Nothing would get rid of it.
Eventually it got so bad that I couldn't see a single fish! I bought a product called "Aquarium conditioning blocks" the next day the water was crystal clear and it's been a week and my fish are fine and the water is still clear.
It is a product that is made in Australia by an Australian company their website is www.cpv.com.au I really recommend it, they worked for me and I was really thinking that they wouldn't. They are only around $4.00 (treats 120L) I had to buy two packets because my tank is 180L.
if the water is only put in a little while ago then its an algea bloom, it goes away by its self, no chemicals needed, happend to me took about 2 weeks to go away