In fairness yellow water is not ALWAYS a sign of unhealthy water...When I went in to buy my first fish the fish store had lovely, clean, well kept tanks and then there was one tank in the corner that had brownish water...i thought to myself "
eek! they've been slacking on that tank" but I didn't say anything and went on to buy the guppies I was wanting.
I said to the lady that I was thinking of getting a catfish and which one would she recommend for my set up...the lady pointed out a few and gave me basic information on them. One of the plecos she mentioned was in that brown tank and she said that if I chose that one I would have to make sure to give it bogwood, and that it could make my tank brown, because of the tannins in the wood, although if I boiled the wood it would minimise this problem.
So, coolfish as you can see does have wood in the tank, so it could be that making the water yellow. I still agree with the others who have commented though that you should really get rid of some of those fish, for their sakes
.