I just bought a new tank, 75gal, about 3 weeks ago. I bought the tank because the Koi and Shubunkin I have got too big for my 35gal tank. I also have a Pleco wich is also good size. I was using a Aquaclear 70 filter and with the new tank they assured me that the Emporer 400 would do the job. Since I got the bigger tank, I also got some more fish, mainly catfish to help with the excess food. But now my fish are sick with tail rot and my nitrite and nitrate levels are through the roof. I also purchased 3 oranda gold fish ( I call them abby's, as in abnormal, don't know the real name for them). Since I have bought the new fish and the new aquarium I have lost 4 cory catfish, 2 Abby's, and two silver tip catfish. Now all I have is a 14inch Koi, 10 inch shubunkin, 8 inch pleco, and one abby. oh yeah I forgot to mention my blue lobster. Anyway, the guy at the pet store gave me some stuff to get rid of the high nitrates, by suggesting I change 50% of my water and put in something called CYCLE, someting that is suppose to help with fish loss, and MELLAFIX, to help with the tail rot. He also said to use the directions as if I were setting up a new tank. Needless to say I lost 5 of the afformentioned fish due to this process. I still feel my main problem is with the filter. When I set it up originally, he told me that I only needed to fill one media filter with the ammo chips. Now I do know that with the original fish that they produce alot of ammonia, that is why I was a little skeptical. Now with my nitrits and nitrates through the roof, I am a little skeptical about the filter I currently have. I checked them again, and although I removed the carbon so the medicine will work, and filled both media filters with ammo chips, there is not a significant difference and the fish I have left are moving rather sluggishly. I am wondering if I should buy a bigger filter, such as the Aqua 110, or maybe another Emporer 400 filter so this won't happen again. Before I had gone 3 weeks without cleaning my tank and never a propblem with the ammonia, Now i clean it faithfully every week (new tank, you understand) and I am replacing 25 gal and still am having a problem. Also, should I keep up with the medicine even if it seems to be making them more sick Should I just start over and empty everything out??? (I mean the water). And would another filter help the process, or maybe a bigger one??? please help me, I am at a loss.