i have just cleaned out my tank for the first time in three weeks due to a burst pipe and behind one of the filters is black gue i had a fish die yesterday and now wonder if he had eaten some of it does any one know what it could be i havent got any pics but i did have the uv filter pointing towards the named filter was just wondering would that cause it i have another filter in the tank and that one was clear. just read this Black Algae in a fish tank isn't actually algae at all. It is bacteria protein called cyanobacteria. If your asking how to rid your tank of this, the first thing you'll want to do is test your water for nitrates and phosphates. Generally this is a major factor in the growth of the bacteria. Scrub the glass, clean the decor, and stir the gravel. Remove as much as possible with a net. Next perform a 25-30% water change. This will help reduce the levels of nitrates and phosphates. Also, since this type of bacteria has to photosynthesize reducing the amount of time the light is on will be greatly beneficial.
<BR style="DISPLAY: block; CLEAR: left">
Read more: http://wiki.answers.com/Q/Black_Algae_in_fish_tank#ixzz1AH9NAKiu