2 tanks, same water but different pH's

[Todd2]

I have two tanks, a 20g long and a 10g. The pH of the 10g is 7.6 and the pH of the 20g is 7-7.2 (I planted it 12 days ago). But I'm using the same water, how can this be? I asked my dad to look at the test kit to make sure my eyes were working and he said the same thing. I'm not adding any chemicals to either tanks. There's no CO2 injection either. I looked back in my fish log to when I was setting up my 10g and to my suprise during cycling the pH was around 7. Then as time went on it stayed stable at 7.6. Anyone know why this is happening? I'm getting Rams and cardinal tetras and I know they're sensitive to water conditions. Will they be alright if I add them in (like in a week) because the pH increases so slowly?

[TankGirl]

Have you tested your tap water? I'd test it straight out of the tap, then after letting it sit in a container at least overnight. See what you get.

Otherwise, a gradual rise in pH often means a buffering material in your tank, like the gravel, sand or decorations. Do you have anything like that?

[poikilotherm]

Yeah, what TG said.

Check your water hardness. Really soft water has practically no buffering capacity. KH (carbonate hardness) is what you'll want at least a bit of to maintain a stable pH.

Just remember that the stability of water parameters is more important than their actual numerical value (except for ammonia, which should be stable and zero!)