I'm wondering whether your water's kH is low and the organic matter in your tank is using it up so the ph drops in your tank after a few days. If u get a kH/gH test kit u can find this out. Our water has low kH so we use little baggies of crushed coral either placed in the filter or sprinkle the crushed coral in the sand/substrate of the tank. This makes sure we don't lose our buffer kH and holds the pH steady, and as long as we don't use too much it doesn't raise our pH etc. Otherwise you may have to do much more frequent water changes to replenish your kH daily.