[lemonnoodle]

Hello all,
I feel like you all would be more knowledgable on this subject. I just set up a tank (ten gallon) and filled it up with RO water; to my surprise the ph still measured to be 8 or above. I'm currently using an inert substrate, black diamond blasting sand, and it did have probably half a gallon of normal tap water in it. The ph of my tap is normally very high but I had expected the water to be at most 7. Any ideas?

 

[DaveK]

RO water has almost 0 mineral content, so there is no buffering. This means that it only takes a trace of something to make the pH swing way high or low. That half gallon of tap water can make all the difference.

You generally need to add some sort of GH builder and KH builder to RO water before using it. At that point, you can test and adjust your pH.

 

[Rob in (ca)]

Like Dave K has stated using 100% R/O water with 0 minerals 0 buffers will harm your fish & plants........ get yourself a KH / GH test kit , add your R/O with some tap to get your desired KH /GH this all depends on what type of fish you have?........ I keep mine at KH 4 and GH 5 I have Discus & Cardinals

once you keep a steady KH /GH your fish will have a safe PH

 

[dmagerl]

In addition to whats already been said, an RO filter alone is only about 95% efficient at filtering. So there will always be a miniscule amount of minerals left in the filtered water which could swing the pH to the high side. Unless you follow the RO filter with a DI filter, then almost everything is removed.