I have had a 75 gal freshwater tank for awhile now. I never do any water changes except for when I have had these kind of problems and I need to know what im missing and what i need to do to fix it. Ok so for starters I have had just random fish die. Some will last for a year some will last less then a week. Out of no where. Never showing any signs. Which in turn makes me have to buy new fish over an over. Another problem I have had and currently do is my fish are swimming to the bottom of the tank and just pointing downwards. Now they will move about the tank at random occasions but then return to their school and face downwards. What I have noticed today is now they are still as can be at the corner of the tank at the top of the tank..... very odd. My bottom feeder tend to freak out every now and then and likes to dig a lot. The diging I find to be normal but now the ziping around the tank and swimming and breathing fast. I know I have had in the past a very slight amonia level but nothing to worry about and has been the same since the begining of this tank. I do not make water changes but do keep my filters clean and add water treatment when I refill the tank. Please help! I have some few lon lasting fish in there I would hate to loose. Maybe if someone can tell me in detail how to stay on top of this tank so I can stop having issues repeat and loosing fish all the time. Or point me to a site. Please and Thanks so much for your help!