We had the gold fish in our tank last year. I think ours is a 200 gallon trough. The gold fish did keep the mosquitoes down, but didn't do much for the algea. I added fresh water every other day. I still had to dump it & clean it every once in a while. The tank gets a lot of debris (hay, dirt, leaves, & stuff) in it. I didn't do it this year, because I ended up cleaning the trough anyway.
I didn't notice changes in the horses when we added the fish or when we removed them.