Well, garlic is a good internal parasite remedy. You just rub some (or use the juice from minced) onto the food and feed it to them. It is also an appetite stimulant.
As far as treating pH, unless you are keeping fish that are very pH specific, don't worry about it. It is far more important to maintain a steady pH, than one that the fish would "prefer." Most pH values are for wild fish anyway, and most of the fish you buy are captive bred and have been acclimated to your water parameters while at the lfs.
Don't try adjusting the pH, the swings harm the fish more than leaving it at a steady, perhaps "too" high or low value, would. Think of pH like humidity to us. We can adjust to high humidity, but its a doozy when it keeps changing from high to low.
Its much better for the fish to stay at, say 8.0, than to bounce between 6.5 and 7.5
But, rather than chemicals, which are unsteady, to lower the pH you could use peat moss in the filter along with driftwood. Crushed coral can be added to filters to raise the pH/kH/gH. But you really shouldn't need any of that

Though tannin stained water has a nice effect and works wonderfully for tetras
