pH keeps creeping up
tank is a low tech 10G with black sand (I think its really just glass) and lots of anacharis, a few anubias and a java fern. I have a 2 different large rocks, one seems to be some type of shale or limestone and the other is volcanic rock, then I have tons of other smaller pieces of petrified wood I have found in local riverbeds (another hobby of mine) all of which were scrubbed clean and boiled to disinfect. I have 10 neon tetras, one neon dwarf gourami and two cory cats. About 2 months ago I began to have a beard algae problem and right about that time got an API master test kit. Tested the pH and read an 8.4, in attempt to keep the algae at bay I did a 10% water change daily and 50% each week using reverse osmosis water bought from the store. (my tap pH is very high) During this time the pH went down to about a 7.6 and stayed the duration of the frequent water changes. I've now left the tank alone the past week and the pH has crept back up to 8.2 All my fish seem to be happy and healthy (they are pretty hardy species) but it still bothers me my tank pH is so far from neutral. Ammonia is 0 and nitrites are at 0. I also have 2 medium sized pieces of driftwood and one that is pretty large for the size tank its in.
Why does my pH just keep creeping back up? Could it be the substrate or one of the rocks? I expected the pH to lower after adding the last (large) piece of driftwood but it has not. This problem has been rather annoying, also, this may be related or unrelated to my issue but i am using hydrogen peroxide in small doses about once a week to kill the dreaded beard algae.
Thanks for any help