I have a 20 gallon currently still going and I used stratum as my substrate. It made my ph go from 7.5 to 7.2, my ph has stayed at 7.2-7.3 for the past few months, I think, and by that i mean i dont know, but i think that after 2 years in MY experience the ph will start to go up as you do water changes and the stratum weakens. Your fish will most likely be fine. Ive taken a family friends guppies, tigerbarbs, corydoras and snails come from their tank with a ph just below 8, they went right into the 20 without issue