pH unstable

[Tenko]

I set up my new tank several weeks ago but have been struggling with the ph. Out of the tap the local water is between 7.5 and 7.8. For the first couple of weeks of starting a fishless cycle the water was 7.8. I added a few fish to help the cycle along and it stayed at 7.8. 6 days later I did a ph test and it was 8.2.

I reduced it back to 7.8 with a few water changes, which I was doing daily anyway, but it kept creeping up. I had a few small shells in the tank which I removed in case they were the cause, this had no effect. I did another check today and the ph is now 8.3.

I have black gravel, rainbow rock, several stones bought from a lfs which knew I was setting up a freshwater tropical tank, 6 plants (mostly ferns) and some decorative columns (which I assume are a type of plaster). Will any of these cause the ph rise? I understand that rocks containing calcium may cause this, how do I tell if any of my stones contain this?

I have an airstone running during the day but no source of CO2. The tank is lightly stocked, as my signiture indicates.

Any advice would be appreciated as I don't want the ph to rise much more and worry that daily water changes simply cause the ph to fluctuate too much to be good for the fish.

 

[FrankSlapperinni]

To see if any of the rocks have calcium, just drop some vinegar on the rocks, if it fizzles, then it contains calcium. Don't worry about adding the rock back to the tank after having vinegar on them, if you rinse them under warm water, the vinegar won't effect you fish.