Massive, Sudden fish loss-Seeking advice

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oups.com...
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



Do you mean you had a 10 gl with fish you had for some time, then you set up
another 10 gl , but when you got new fish for it you put them in the old 10
gl w/ the older fish instead of the new 10 gl?
Did you put the new fish in QT for any amount of time? if not maybe you
introduced a disease that the new fish had into the old tank, the fish in
the new tank may have been infected with the same thing as the others you
put in the old tank.
I have got guppies and found out after the fact they were sick and there for
lost some older fish I had because I did not wait to put them in the tank, I
always learn the hard way, it took a month to resolve.... the white fuzzy
stuff was probably fungus, my guppies I bought had it also, if you only seen
it after they died I don't think that matters but if it was on them while
still alive then that is probably what it is. I lost several of my older
guppies before I got it taken care of, are all your fish dead? if not I used
aquarium salt, fungus treatment and a lot of water changes, but someone else
might know better then me.
I am not sure if fungus will stay in the tank if you completely clean it
out, maybe let it dry in the sun, I'm not real sure, I would throw the rocks
away if they are just cheap rocks. I boiled all my stuff after that
happened.
Nik