I think your heatsink is copper. I saw many poisioning fish case because of little copper lines on LED stripes. Water evapourate ofcourse, and oxidate to copper. Not fast but slow and strong. And little tiny copper-oxides powders starts to fall on water... And it's certain death to your fishes as you know.
So i recomend use a evapouration glass or better way change your heatsinks to aluminium ones.
I hope it will help. Please think about it.
Turkish boy from Hong Kong...