FishProfiles.com Message Forums

You probably won't need all that much RO to bring the pH down by 1 point only. Pure RO water lacks natural minerals and salts that are necessary for osmotic regulation and other equally boring processes in fish, so it needs to be reconstituded with a commercial preparation if you want to use it alone. Unless you're living in a toxic waste dump, that really isn't necessary.

Try fiddling with it to get it where you want it, I don't think I'd go with less than 1/4 tap water if I were mixing it but some people do more. I would probably mix an easy number like 1/4 or 1/2 RO just so I would always remember how much to use, but the best method of finding out how much to mix is really by experimentation. Mix up a few small buckets of water with different % RO and test for pH, GH, KH etc. Keep track of the % mix in each bucket, leave them to age overnight and test again just in case dissolved CO2 is going to make any difference. When you find a % mix that you like, slow acclimate your fish to the new conditions over a few weeks. Topping off due to evaporation should be 100% RO water, as minerals and other junk will concentrate in your water as it evaporates.

Of course if all of this is too much of a hassle, you can always toss some peat in your filter and forget about it. If you're super picky and don't like tea water, RO is the way to go.