What is wrong with this rosy minnow?

Question

I'm done cycling my tank, an I used rosy red feeder minnows, because they are very hardy and cheap. Try all live through the whole cycle without showing any signs of sickness, except from about yesterday, an adult sized minnow started acting very strangely. He is (right now) struggling to stay under water, because when he relaxes, his whole body floats to the top. He struggles to not float, and I was wondering if this is some kind I disease? I'm returning all of them on Saturday to get my puffer, btw, so I'm not keeping him as a pet either, I'm just curious. It looks like there is a giant air bubble in him and he keeps floating to the top if he doesn't try really hard to swim against his "floatingness"

Answer 1

Best Answer - Chosen by Asker

They may have temporarily lived through he cycle but not without impacts as you are seeing. Poor water quality doesn't have to kill a fish outright...it can cause irreparable damage to internal organs that are fatal in the long term. Likely the minnow has such an issue, something internal directly caused by the poor water quality experienced in the cycle.

Asker's Rating:

Asker's Comment:

Actually, this answer would have been informative but I found the right answer after an hour of researching, or was because the lack of oxygen (I don't have air pump, only filter that supports 5 minnows in 10 gallon) but thanks for the estimation

Answer 2

They are going to die you need to cycle the tank again