Join Date: Feb 2013
Location: West Palm Beach, FL
I don't think anyone can give you a definite number. There are too many variables. I would just get a piece that looks good in your tank and pop it in.
In addition to IAL and driftwood, alder cones can lower ph. Peat moss works too. Put some in a bag made out of stockings, or any fine mesh nylon, and put it in your filter. Change out the moss every few weeks to a month.
Good luck! I have a crowntail in similar pH level water, and his fins are fine. But they are all different, and the effects of water chemistry are complex.