I have used this product and have seen no ill effects with fish. If running carbon in your filter however, it will quickly be removed by the carbon. Considering weekly water changes ,it has little to no impact on pH. I have found that driftwood that hasn't been boiled for too long ,will achieve the same effect. Both the product, and the driftwood will turn the water a tea color due to tannins relased. When I used it with blue rams,,it did seem to have a calming affect on the fish but perhaps ,,it was my imagination.
Driftwood would be cheaper in the long run.