The best PH money can buy, IMO
, is the Maxi Jet. How many? Depends on what kind of water flow you already have. You need at least, for a 90, 900 gph
. The Maxi Jet 1200 is rated @ 295 gph
. So, in theory, 3 of them would put you about right. The 900 is rated @ 230 gph
. If you put one at each end of the tank and one blowing behind the rock, you should have plenty of current.
I'm not saying that other PH's are no good, but the MJ is the best of the bunch.