Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Either RO or distilled is fine. They are pretty much the same as far as aquariums go.
If you will be blending a little tap water into the mix to get the minerals then you will need dechlor only for that portion of the volume.
I would do something like this:
Set up the water for water changes the day before.
Blend whatever ratio of tap + RO (or distilled) to get the mineral levels you want, and add dechlor based on the amount of tap water.
Add a bubbler or small pump (table top fountain pump is more than enough for this small amount), and let it run overnight.
This way, the RO or Distilled has a chance to come to equilibrium with the air. Oxygen, CO2 and atmospheric nitrogen levels will stabilize, and become equal with the tank water.
Do the change slowly, perhaps 1 gallon at a time every 2-3 days so the mineral level does not drop too fast. The fish will take some time to acclimate to the softer water.
If the tank needs a bigger water change than this, then make up some water the matches the tank water GH, KH, TDS, pH. Do as big a water change as needed with this sort of water.