Help and advice

Well I hope I have found somewhere that I can get some serious answers
a lot of places either won't answer you or tell you off. My problem I
have a small 500 gallon pond that is only 2 feet deep. I have a 650
gallon pump in it. This pond is in direct sun and my water of course is
staying green. On this small of a pond which has no fish btw... is a UV
Sterilizer Bio Filter really needed or are there chemicals to clear up
my problem? I had a few goldfish in there until we added algaefix,
afterwards within a day the fish were dead.