Sounds like fun Cecil (not). Probably have to adjust every time you top up, wait, and then adjust again before adding the water to your system. Keep at it
Cheers
Shell grit is probably the easiest way. Use a removable mesh bag so you can take it back out if the pH starts to go too high. If your pH isn't coming back up or if you think you need an extra kick to get the pH started upward then adjust your top up water NOT the fish tank. The pH should change no more than 0.4 pH points (although some fish can take way more than this don't push your luck). Calcium Hydroxide, Potassium Hydroxide and Potassium bicarbonate will all work for adjusting the pH up. The Hydroxides are stronger bases than the bicarbonate. The bicarbonate will add more buffering capacity though so they each have their advantages. Information on amounts to use is guesswork because the buffering capacity of your water really determines how much you need. Adjust the top water only.
Potassium Hydroxide (KOH) - Used to raise pH and add potassium. 1 T / 1000L every few weeks.
Calcium Hydroxide (Ca(OH)2) (Hydrated lime or Brickie's lime or slaked lime) - Used sparingly and with caution (caustic) to raise pH and add Calcium. Probably adding about 1 tablespoon per 1000L until you figure out what works for your system is a good start. Remember not to change the pH more than .4.
Potassium Bicarbonate - I don't have an amount listed for this so can't help you with this. I think it will be more than used for the hydroxides even though it adds buffering capacity but haven't tried it.
Shell Grit (one form of CaCO3 (calcium Carbonate))- Used to raise pH and add Calcium (Put in a cloth bag so it can be removed from the system, hang in sump or place under outlet)