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.

I should also say - I kept an eye on pH during the fishless cycle and although it crashed at one point, it never went above 7.6. So I assume the cycle was masking the effect that the elderly stone was having?

Sorry to read about this. I can only reply briefly but:
* think you've got the right approach ie small, regular water changes;
* not convinced the elderly stone or the 15% would have caused the deaths - any contribution to the deaths would have been very small;
* what is your water hardness? If you don't have a liquid-based water testing kit for KH and GH, what does your water supplier's website state as the levels of CaCO3 mg/l or German degrees hardness? [Guppies' requirements are >143 ppm/CaCO3 https://www.seriouslyfish.com/species/ ... ulata/?sfw=pass1614789693]

Thanks for your reply! I don't currently have testing kits for KH and GH, but I checked the water board's local report (which is currently offline, annoyingly) before I got the fish and the water is moderately hard, but not excessively so. I used to keep fish in London and the water isn't as hard as it is there. I'll keep trying to access the report.

My LFS is fairly local so I am guessing their pH is more or less the same as my tap water (about 7.6) - but as my tank is higher than that at the moment (8.2) I'm just wondering if it has shocked them every time I've done a water change?

The last one which I thought was a goner seems to have rallied, and the rest are looking healthy, so that's something!