Never came across such an episode in my 2 years of fishkeeping. I had 3 guppies in my 2ft tank together with some plants. After acclimitisation for a week decided to inject co2. Indicator shows dark green (indication of an adequate co2 level). After a few hours saw all the guppies drifting upside down. Proceeded to remove the dead fishes. Scooped up one with the net threw it out of the window into the garden. When i was about to scoop the second one noticed it was still breathing. Felt bad about throwing it away and left it to have its last breath. Next morning was surprised to see the remaining 2 guppies swimming happily and feeding.

Again checked on them about the same time that they were spoted drifting upside down. Again could see an upsidedown drifting almost lifeless guppies. Tried touching them and there was no reaction. Nevertheless left them as it is and was hoping for a miracle next morning. Again to my surprising the 2 guppies were happily swimming.

Just to make sure my eyes are not playing tricks on me decided to call my brother to witness the upside down drifting guppies on the 3rd day. He also claim that they are dead for sure. The next morning again they were swimming with no sign of any side effect.

I witness the same thing for the next 2days but decided to tune down the co2 level as i dont feel good about it. After that the upside down swimming stopped. I know it has something to do with co2 level but just curious to know if anyone had similar experience.