The problem with RO is that you kH (carbonate hardness) is going to be very low - so you will still see wild pH swings.
I think what we need to look at to resolve this problem is to increase the kH and GH - this will naturally increase the pH but if your pH is dropping off - by increasing the kH and GH things should be more stable vs. adding removing coral which is just effecting the pH.
The higher the kH of the water - the pH should not swing back and forth.
I'm lifting this from a google search:
Ways to lower pH
Filtering water over peat
Add bogwood to the tank
Inject carbon dioxide CO2
Use a commercial acid buffer
Water changes with softened water or RO (Reverse Osmosis) water
Ways to raise the pH
Aerate the water, driving off the carbon dioxide (CO2)
Filter over coral or limestone
Add rocks containing limestone to the tank or use a coral sand substrate
Use a commercial alkaline buffer
Ways to increase kH
Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the kH of the water by approx 4 deg dH without a major affect on pH.
Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)
Adding commercially available products to increase buffering capacity
Ways to lower kH
Injecting carbon dioxide (CO2)
Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired kH.
Adding commercially available products to decrease the buffering capacity.
Ways to increase gH
Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
Adding calcium carbonate will raise gH and kH
Ways to reduce gH
Adding peat moss to your filter
Use commercially available water softening pillows or a water softener
Mixing tap water with reverse osmosis (RO) water.
A long but interesting read is here:
GH/kH and pH
This article is from another forum but I think it is very relevant to understand your situation - so hope thats ok with the mods.
