To best answer this question it's always a good idea to start at the source first. You want to test your source water for pH but even more importantly the gH. If your source water has a low gH (under 8), that means that it's much easier for your pH to drop. Especially if this is a tank with larger fish that you feed heavily, it will make the pH drop faster, being that theres more organic waste present. Driftwood or peat moss will definitely drop pH so as long as you don't have that in your filter no other media should lower it. If you have some extra room in your new canister filter, a way to counter-act the pH dropping would be to add a bag of crushed coral. If that's not possible you could also add the crushed coral to your gravel bed. If you have a natural colored gravel it generally mixes in without a trace.
