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Crazy pH in fry tank!

Subject: Crazy pH in fry tank!
by Shubunkin72 on 2018/12/22 14:05:36

I noticed today that one of my 6 week old fry has one gill curled up, I didn't notice it yesterday and I check on them a lot. I do a daily vacuum of any fish poo and any leftover food, and also change 60% of their water. As they were unexpected it was a new tank set up and therefore wasn't cycled, so I added some plants from my main tank and some of the filter media and hoped that combined with the use of Prime it would be ok.
I did notice a rise in ammonia and then nitrite, and finally was at 5.0 on the nitrates, and some days did two 70 % water changes to keep ammonia and nitrite levels down. After reading up on the cause of gill curl, I quickly did an ammonia test and was shocked that it was green and around 1.0! The nitrite was also at 0.50, and nitrates at around 5.0. I thought my tank was cycled about a week ago as everything seemed fine by then. I also did a pH test, I hadn't been doing them as I guess I was more focused on feeding, cleaning and checking other things.
The pH was crazy, the colour in the container was bright yellow, and the lowest reading on the card is 6.0, and that is a lime green! I did have problems with my main tank, my tap water is 7.4, but within an hour of being in the tank was dropping to 6.2!! I tried adding more rocks, and eventually tried crushed coral in the filter, which worked and things settled down. But this is even worse! I have a doctorate in molecular biology and I'm at a complete loss as to how water can drop from 7.4 to below 6.0 in an empty glass tank! The only things in there are the filter, heater, and some anacharis from the main tank. I've just added some rocks from the main tank in the hope it will help, but I'm utterly confused!
I assume that the low pH must have caused the death/dormancy of the bacteria that had established as I can't see why else I have such high ammonia levels. I tested the pH with the Prime and that actually raised the pH to 7.8, so it's actually a drop from 7.8 to down below 6.0 within an hour...my poor little grandfish! And my poor Obi (the biggest fry) and his folded back gill...any advice will be gratefully received!
Thank you!