the tank is a 75 g. its an older tank that has been setup at another house for i would say 3years. i got the tank and emptied the water, rinsed the sub straight set it backup. its been 2 years now that i have had it.
it is a well planted tank and has a large piece of driftwood with more than enough fish. my question is, for the last 6 months i have noticed my PH has be slowly getting higher. all other parameters are fine. i use tap water and treat it with prime. out of the tap the water is 7.0 and has never been more than 7.2, and in the tank its 7.6 to 7.8. i have used some Ph down over several days and it came down to 7.2 but 24 hrs later it shot back to 7.6. I have ecocomplete and the old river rock as sub straight and i keep the nitrate level at 10 to 15 (the plants seem to like it better there) and i do a 40% water change every two weeks. any ideas whats doing this?
any help is appreciated. the local fish stores are no help. thanks in advance.