Let me start with the basics: lightly planted 29 gallon tank that has been in operation for several months and has had no deaths, or even signs of disease, until now. It contained (before the recent deaths) two ~2.5" gouramis, 5 guppies (2 male, 3 female), one male betta, and one 3" plecostomus. The water parameters are 0 ammonia/nitrite, ~20ppm nitrate (can't get it lower than this because it is high in my tap water), pH ~ 7.2, water temp 77 F.
All but the guppies have been in the tank almost since the beginning. I introduced the guppies on 10/17, and when I woke up in the morning on 10/20, one of the males was dead and stuck to the filter intake. Then yesterday evening (10/23) I found the second male swimming vertically and he died almost immediately. I was slightly alarmed at this point because I inspect my fish every day when I feed them for damaged fins, discoloration, lack of appetite, etc., and all of the fish seemed healthy less than a day before. However, I chalked it up to acclimation difficulties, since they were new fish.
But then tonight when I arrived home from work I went to the tank to give them a light feeding and my paradise gourami (Pinky) did not eat. I gave a closer look and her/his (?) fins were badly damaged and (s)he looked like she wasn't breathing and was making occasional seizure-like movements. I had fed this tank not more than six hours earlier and (s)he looked just fine! At first I thought maybe another fish had attacked, but she and the other gourami are the biggest fish in the tank, and she has always been dominant besides. (Not to mention: no damage to any of the other fish.)
It looks like there is some kind of intestinal parasite coming out of Pinky. I did notice yesterday that (s)he was blowing occasional air bubbles ... which is not completely abnormal, but they were more numerous and bigger than usual. Could that have been a warning sign?
Anyway, I am not used to losing fish -- especially not this suddenly. I have never dealt with any disease other than Ich before. I am also not necessarily convinced that what killed the guppies is the same as what killed my paradise fish. Can anyone offer a better explanation? Or, better still, treatment advice (could the rest of the tank's residents be in danger?)
I took some pictures, which are available at this address: http://tastybrains.dyndns.org/dying/dying.html
Any suggestions would be appreciated. Pinky had a really cool personality, and was by far my favorite fish.
P.S. I know it's a little late for this now, but can anyone tell me for sure what Pinky's gender was?