Let me start off with a little background...
I had 9 glofish (danio), some corys, a bolivian and a blue ram in my 20gal long tank.
I have had my glofish and corys since January (8 months) and have never had one die. They went through 3 tank changes and were my starter fish, so these were extremely hardy fish. I've had my rams for about 2 months.
It had been about 2 months since my last filter change, so last weekend I replaced the filter and did about a 33% water change. I keep my pH at 7. When I replaced the water, I put way too much acid buffer in and didn't test it. Well the next day my bolivian ram started looking very sick and I panicked. I went to the LFS and tested my water. They told me my pH was off the charts low (some where around 5). The weird thing was no other fish seemed sick.
So I rushed home and added a scoop of Alkaline buffer to raise the pH. Keep in mind the only sick fish was the Bolivian at the time. As soon as I added the Alkaline, all the glo fish shot to the top and were in the corner of the tank gasping for air. My pH was only raised to about 6.4, so it wasn't too big of a jump. I now have a test kit and have tested twice a day. My first question is why did the glofish shoot to the top and have trouble breathing from the Alkaline buffer?
Over the next few days I did about 20% water changes until I got the pH to its current level of 6.8. I have had 5 glo fish die, one every day since Sunday, and two more are sick.
The weirdest thing is two of the glo fish never went to the top and gasped for air. They acted like nothing has changed and are completely healthy. Also, the bolivian got healthy as soon as I fixed the pH. It blows my mind the bolivian and blue ram have been thriving through this all, while my hardy glo fish are dropping daily.
Why did this kill all of my glo fish? The only thing I can think of is they can't live in the acidic water of below 7 pH since they have been at or above 7 their whole life???
Any help is appreciated.
* Also, when I changed the filter I only replaced the white pad and put new carbon in it. I didn't clean the sponge or the inside of the filter. It is a Top Fin 30 rated power filter.