I have a 50 gal freshwater tank with 4 drawf rainbows, 6 runny-nose tetra, 2 angel rams (drawf cichlid hybrid), 1 female guppy, 3 platy, and 2 gold dojo loaches. Most of these fish I have had for 6-7 months. I had a 44 gal tank that cracked and so I had to buy the new 50 gal tank and transfer all the fish almost 3 months ago. I didn't have any deaths after transfering all the fish and everything has been great until about 2 weeks ago.
I noticed that one of the drawf rainbows was bloating and not eatting. I thought that perhaps I was getting over zealous with the food and that I was overfeeding. I backed off and made sure that I fasted the fish once a week. One buy one, 3 of my rainbows swelled up and died. Now another rainbow is so skinny and he can't swim right. It's like he almost has swim bladder but not all the way. Today, a healthy looking tetra died, and about a week ago one of my platy fish kept "relaxing" in the bottom corner of the tank. Then a day later it died too. I also had my brand new pleco die after having him for only a week and a half.
I have never had such terrible problems with fish dying. I recently performed a water change because nitrates where just under 40ppm. After I changed the water they have gone down to around 10ppm. No ammonia, no nitrites, and pH always seems to be 7.5.
I always look at the fish after they have died and none of them have any "damage" on the body. Some of them look just as good as when they were alive. Did I pick up a disease from the fish store? Does anyone have any ideas how I can find out what is killing all my fish? How do I treat it?