My son got a Juwel 70L tank for Christmas. We did a fishless cycle and brought home 9 guppies last Thursday. We lost one on Friday, one on Sunday and two today (Tuesday).
Ammonia and nitrite have been 0 since we introduced fish, nitrates are currently around 10ppm or slightly lower. I am wondering if we're losing them because of osmotic shock.
We checked the tank parameters before we left for the LFS and the pH was 7.8 - it comes out of the tap here at 7 and rises to 7.6-7.8 within 24 hours. We lost the first fish a day later which I put down to travel stress - we did the usual floating the bag etc but I know it's a stressful process.
Within a few days the nitrates seemed to go up from 5ppm to 20ppm (though I find the API test hard to read and have a different one on the way) so I did a 20% water change. We lost another the day after. Really stupidly, I didn't check the pH of the tank before I did the water change - I just assumed it was unchanged.
We then lost another fish and a third started looking sickly. I checked the pH of the tank yesterday and it was 8.2 - having done some research, I'd put elderly stone in the tank and didn't realise it would increase the hardness and pH, so I removed that. I now suspect I did the worst thing I could do and did a 15% water change to bring the pH down to 8, and lost another one this morning.
The pH has returned to 8.2 today despite the water change. I started doing tiny, period water changes this morning - just a couple of litres at a time well mixed with tank water, and I've lost another one.
Do you think this is osmotic shock? Am I doing the right thing in doing small, constant water changes to start bring the pH down? I used to keep fish and have never lost them in these numbers and I'm horrified, so any help is much appreciated.