Water samples which indicate a high pH level when tested, could be adjusted by straining water through peat. The instructor at BCC compared it to "tea," which is dark, but safe for fish. Added to the aquarium, the pH level was brought down within a safe range.
Here's John straining the brackish water.
Large debris are removed by hand, then strained through a fine net tightened down with a bungee cord (pulley) on a large bucket.
Brackish water, (low pH solution added to one of the empty tanks). John (instructor of Aquaculture Practicum at BCC) warned it wouldn't be good for fish to be swimming in their tank and suddenly the pH drops an entire point, so the transition is accomplished gradually.