Join Date: Jan 2011
Location: Oklahoma City, OK
Yeah, it doesn't quite work the way you were planning.
Water has a pH of 7.0
RO water is fairly pure. If you put your TDS meter in there you will probably measure something fairly low. The pH of your RO water is probably measuring 6.4 because of normal dissolved gases(i.e. CO2). This is because RO water is very sensitive. There is nothing but H20 in there, so even the slightest addition will change the pH. For all practical purposes your RO water has a pH of 7.0 unless you do a TDS and it tests high.
When you mix your RO water with your tap water, the tap water is full of dissolved solids. These solids are alkaline. Pure water + dirty water = dirty water. So, you mixed 7.0 water with 8.5 water and got 8.2 water.
If you want your water to go below 7, you need to do two things:
1) Remove the alkaline "stuff"
2) Add acidic "stuff"
(You could try to just add a lot of acidic stuff, but you are going to wind up with brown water)
The best way to do this is to only add RO water during your water changes for awhile. If you are doing this for fish, you might try fullerized clay(cheap and readily available substrate). Fullerized clay will pull all the stuff out of your water column and bind it to the clay.
As a side note, you can go pretty low on the pH with cardinal tetras. Battery acid low(lower than 5.0). Neon/Cardinal tetras have some of the highest known tolerances to acidic waters.