I have a 240l planted tank which has been running for almost 3 years. I confess I have not been taking water chemistry readings for the last year until recent fish losses. I have just bought a master test kit and the readings were : Ammonia - 0, Nitrate - 0, Nitrite -0 (all as expected in a well matured heavily planted tank). The pH however has got up to 8.2, I am certain that this is the cause of death of the fish and am taking steps to correct it. What I don't understand is how this has happened. I do regular water changes with 50:50 R.O and tap water. Our tap water is pH 7.5. There are 4 large pieces of bog wood in the tank and no rocks. The gravel is lime free. To back this up I tested the kH and it is less than 2. Why has the pH gone up? and what should I do to bring it down slowly to between 6.5 and 7.0 - I think buffers may fix soluble nutrients and make them unavailable to the plants? Any help would be much appreciated.