OK, I've got a Tilapia disease problem, that I can't find an explanation for as far as the symptoms. I've checked all of the links in the post by JCO for the symptoms Tilapia Diseases & Cures, and none fit at all, so I'm guessing it's something else.

The fish affected are both younger and older, they swim upside down and backwards, don't seem to eat, and this only lasts for a day or two and they are gone. This is only happening in one of the tanks, the others seem fine. All readings are normally good, but I have to change 50% of the water twice a week because the ammonia builds up fast, for some reason. I only have about 6 left in a 30 gallon tank (The other 30 gallon tank has about 16 fish), so I don't think they are stressed by overcrowding. These are some of the last of the Mossambicas that I bought this summer and this fall - I was hoping that out of 100 fry, I would end up with at least one female and one male, but none of them seem the slightest bit interested in breeding. I'm just about to the point that I want to just destroy them all and chlorinate the tank to get rid of whatever is causing the problem, but I don't know if it's a parasite or not and I'd like to know for sure, both for my knowledge and for others who might run up against the same problem. Any ideas what the problem is?

I will soon be destroying the ones in that tank anyways, I guess, because by this spring I intend to have a different strain of fish to breed, hopefully to raise in the big tank! And I won't eat a fish that I suspect may be diseased. Some of the ones in the other tank are just about eating size, I think, so they will go into the freezer .

TIA