Hydro in general is 6 to 6.5 with the only plant I know liking lower is lettuce at 5.8 I got this from Hydroponics: a practical guide for the soilless grower by Benton Jones at the Univ. of Ga. I asked Howard Resh, an internationally recognized hydro grower, and he said 6 to 6.5 with the lettuce exclusion too. I like 6 ph for hydro.treble said:I dont go much below 6.5 and I doubt I could hold it at 5.8 - 6.0 in my res... I dont know but if its correct I will be prepared to try... need to see what the concensus is and look around some more