7 is borderline low for goldfish. They do better in hard, high ph water. 6.2 is very acidic not only for your goldfish but your tank in general as your good bacteria function poorly in acidic water. Once it hits @6, they no longer process ammonia and it will spike. Have you checked your ammonia levels?
Please correct me if I am wrong here but I believe you mentioned in your unhealthy fish post that you only change the water 2x a month. What happens in an overstocked tank with big messy fish and a lack of sufficient wcs is the tank will crash. I believe I mentioned the ph dropping as issue in your other thread. Sorry I missed your post on this.
Basically, your tap only has 'x' GH
levels. They work together to keep ph stable with KH
playing the larger part. Your good bacteria use KH
to process ammonia. Your messy fish are producing a lot of ammonia. And the KH
is being utilized quickly. Nitrates are likely increasing at a tremendous rate as well. Only water changes can restore lost KH
and remove nitrates. Without lots of big, frequent wcs to keep the tank stable, it crashes.
Short term remedy is to increase your wcs big time. Adding some crushed coral or aragonite will help too but long term solution is a much bigger tank for these guys as your tank is beyond its biological capacity. Even extreme filtration can not make up for lack of water volume with messy fish, unfortunately. Hope this makes some sense!