Check to make sure nothing is leaking electricity into the tank. Take a razor and cut your cutical just a bit and dip your hand in the tank. Sounds crazy but I do it if my fish are showing signs of stress and can't figure out why. Had to do it too my Silk's tank last week and found my titanium heater leaking current. You won't feel anything unless you have a cut on you. I have been doing it for years that way. The wife thinks I'm crazy but it really does work and its like a 9 volt on your tounge. Power heads are famous for leaking current