Zebra Danios keep dying!

[Janelle]

I have a 55 gallon aquarium with 2 clown loaches, 8 (and counting down) zebra danios, 3 head and tail light tetras, one pleco, 3 mollies, and a big black moor. The amonia, nitrite and nitrate level is at 0 and the ph is about 7.5. Over the past week, I've noticed my zebra danios, one by one, swimming upside down. As soon as I notice this I put it in a hospital tank, which usually has a male betta in it which doesn't seem to bother them. Usually within a few hours the danio ends up dead. All other fish in the tank are fine, and I have been using melafix for 3 days now to help, but another zebra died today. I don't know what the problem could be. Maybe they are being picked on by the tetras or the mollie? I have no idea. Any one have any clues?

 

[millym]

hi there,
we've had a couple of danios with very similar problems.. one started swimming just a bit off balance & ended up with no control at all- upside down spinning around...poor thing, we euthanased it as the other fish were harassing it. upside down swimming is probably swim bladder infection which you can get medication for from your lfs...but i think its quite hard to treat successfully. we also had a couple of other danios who did the same - just looked a bit wobbly, we treated for internal bacteria(med from lfs...dont know where you are as uk meds are different to usa to tell you what we used) but these danios seem to have recovered so it seems that was what they were suffering from. if yours have been upside down it sounds more like swimbladder though. fish can also sometimes have swimbladder probs from overeating and can recover if this is the cause. danios are supposed to be very hardly but out of all our fish we seem to have had more probs with them than any other species for some unknown reason.
do a search on this forum about swimbladder/internal infection you'll get loads of other posts on the subject.
good luck