Hmm, when did you last flush out the dead vegetation and sludge that always collects at the bottom of the tank and ■■■■. That can put up the acidity too.
I’ve been thinking about ammonia and acidity etc for the first time in a couple of years having just been purging our pond of bottom rot again today, because suddenly this morning we had a big surprise when realised that as well as the frog we have about thirty tiny goldfish just a couple of months old. This is a genuine happy surprise because the neighbour’s three-legged deaf cat had fished out the last adult early in 2014 so we took that opportunity of a fish-free pond to clean it out and re-plant, and I can’t imagine where the fish eggs had been hidden or believe that they could be dormant for so long.