Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Topping off is rarely more than 10% of the volume of the tank:
If a 10 gallon tank is running low I would hope it would not need as much as 1 gallon to bring it back up.
do the test with 1 cup of aquarium water and 2 tablespoons of RO/DI. This is more than 10%, so it is a 'worst case' scenario.
1 tablespoon added to 1 cup would represent about .5 gallon top off in a 10 gallon tank.
RO/DI has no buffer.
When it first comes out of the filter some of the electrons may be stripped from the water, leaving some H+ in excess. This will mean low pH when you test it.
Then, this water can pick up CO2 from the air, and this also drops the pH. Net result may be water that will test very acidic.
Use it as is (and not use very much at one time)
Allow it to sit, aerate it for perhaps half an hour, then see if it is any better.
Add a little aquarium water to it (perhaps make a 50/50 blend) then top off the tank with this (which will be a bit closer to the tank parameters).