Warning: very long post.
1st I'll give you the complete history on this tank.
75 gallon tank I bought 6 years ago. Previous owner tried to keep African Cichlids but kept killing them. After a couple hundred dollars in fish he gave up.
The tank has older 350 magnum canister filter with ceramic and carbon media also a HOB
biowheel penguin 300 with filter and charcoal. Gravel bottom with under gravel filtration with 4 bubblers.
I got the tank 5 years ago when My goldfish were getting too big for the tank and cleaning the tank became a 2-3X a week chore. 3 large goldfish did well in there for 3+ years had a Ph spike and lost 2. The last Comet GF got to be about 8" and also too big for the tank and sent him out to the farm koi pond. That was about 18 months ago. I kept the filters and bubblers going in the tank after doing a good 50% water change and gravel cleaning. and twice in the 18 mot=nths I cleaned the gravel and did a 50% water change.
Fast forward to about 3 weeks ago I wanted to get the water ready for tropical fish. Checked the chemistry and here is what it read:
Did a 50% water change used Seachem Prime to treat the water, double dose. Water here is real hard and treated heavy with chloramines.
waited 24 hours and rechecked water only changes
ammonia slightly above .25ppm
Now I decided to checked my water at tap Ph 7.8 ammonia(chloramines) 10ppm My ph wasn't coming down on its own.
So I do a complete tank drain and cleanout below the gravel filters and everything. kept the biowheels in tank water. double dosed with prime again. and waited 4 days. ammonia was at .25ppm and ph jumped back up to 8.2. Now I still don't know WTH is going on with this tank.
I tried a whole bottle of API Ph down and barely moved the Ph and only temporary.down to 7.8 next day back up to 8.2
I read up on lowering Ph and with no fish yet. I could do drastic measures. so I used vinegar to try to drop it and after triple dosing over 5 days ph would only temporarily drop to 7.8 then it came back up. Now I'm pissed I drop in a 4X dose on vinegar and Ph dropped to 7.4 next morning a complete fog bank in the tank. I suspect carbon dioxide from the reactions of the vinegar. Waited 3 days and started to clear and checked water and
Something in tank is buffering the Ph. Now I am getting crazy about getting this Ph in check. I buy a reversed Osmosis system for our drinking water (I didn't tell the wife also for the tank). I spend 3 days storing 30 gallons of water for a change out and yesterday morning I did a 30 gallon water change. Water chemistry now:
Also I forgot to mention previous owner had the Charcoal media filter blowinto the tank so in the gravel is mixed with some charcoal. Could that be buffering my Ph so anything I do I would have to do triple to make anything stick?
Do I keep doing water changes until it drops? Do I drain the whole tank and replace the gravel?
I'd love help. I really want fish in this tank but DO NOT want to kill anything if I can help it.