50 is still not low enough, get it down to zero. 20 would be ok but zero is better. To give you a better analogy of what you are doing: Suppose you ere in a sealed room, no air in, no air out. You would breath the air in and your body would absorb some oxygen out of the air. You would exhale carbod dioxide and also some oxygen. Slowly your body would use up all the oxygen until you have nothing but carbon dioxide in the room. You would live for a while but slowly suffocate. That is what you are doing to your fish.