wrote:
I had a nice stable 10 gallon tank with 10 fish- 5 tetras, a couple
barbs, zebra danios. The fish had live several years and the tank
looked good with regular siphoning of the bottom and water changes.

The trouble started when I bought a new tank. I started it up and a
week later bought some fish. To introduce them slowly into the new
tank, I put a few into the old tank for a couple of days.

A couple of days later, all 5 neon tetras died overnight. over the next
couple of days, the rest of my fish died. Same thing in the new tank
which was mostly guppies.

Before the hardier ones died, they swam frantically, and often swam
upside down or vertically. When dead they sank to the bottom instead
of floating on the surface. Some of the dying were covered in white
"fuzz", but not all.

I check the pH and Nitrates, Nitrites and they were fine

Two questions: 1) Anyone know what might have caused this?
2) Any advice on how to clean the tank/ what to do before introducing
new fish?
Thanks

Sorry to hear about that. It sounds like a pretty severe viral or
bacterial disease introduced with the new fish. The white "fuzz" could
have just been a secondary fungal infection, or a bacterial infection by
Flavobacterium columnaris. Both are common and you can't tell without a
microscope.

I would clean the new tank out completely with 1:20 bleach:water, rinse
thoroughly with water, dry everything (gets rid of bleach residue) and
set it up again. Rinse anything you can't dry thoroughly in some water
with dechlorinator in it.

I'm not sure what to tell you for the old tank. I would try a chemical
called potassium permanganate to reduce the bacterial load without
killing off my biofilter or hurting the fish. Get Kordon's Permoxyn or
Jungle's Clear Water and follow the instructions. Hopefully it will be
strong enough.

Always isolate new fish from your healthy ones by setting up a separate,
small quarantine tank. It doesn't have to be fancy. I use an empty
tank with a handful of anacharis or some plastic plants, my spare
heater, and a small power filter with a sponge or some carbon from an
established tank. No lights, gravel, or fancy decorations. You can
even use a bucket but it's harder to see whether the fish is healthy.
Some people say to isolate for a week, others say a month. I go 2-3
weeks myself.

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