Cork is good
I for one have used cork bark in tanks before and have noticed NO changes in water chemistry due to the cork bark.
I started with and established/cycled tank, PH was stable at 7.2. I then removed all fish (Cichlids, tropheus Ikolas) and then added the cork bark as a background. Since there were no fish in the tank, I did not do any water changes for 1 month. Weekly testing showed NO changes in hardness (GH or KH) or PH.
After 1 month, I re-introduced 10 Tropheus Ikolas, (55 gallon tank). The PH then began to drop. I had to use LARGE amounts (larger that without the Tropheus) of Sodium Bicarboante to buffer the water in order to suspened the PH drop. This PH drop accured in the first week after adding the fish.
I attribute this PH drop to the fish and NOT the cork bark because of the simple fact that the tank was STABLE for 1 month without any fish and the PH drop accured within 1 week of the addition of the fish.
The first water change was done after the fish had been re-introduced for two weeks. (50% water change) The fish LOVED it, but no difference in test results. I waited two days and did another 50% water change, at that time, the PH came back up to 7.2(It was 6.8 prior to that)
From this test that I did (and BTW, I did NUMEROUS test involving cichlids with plants) I concluded that the cork bark DID NOT effect the water chemistry at all. There is something involved in the process of the nitrogen cycle that causes the PH to drop if not provided with adequate buffering agents. Just exactly what is causing this drop is what I am currently working on in my chemistry class.
The simple answer to your question of bark in the tank?
GO FOR IT.