Just bought RO system..Nows the dumb question. I just got 5 gallons of filtered water. My KH is 1 and my GH is 1. How do i check the PH. I put the RO water in a 5ml and tried to check the PH. The instructions said to add 5 drops of Phenol Red and shake it up and compare the color to a chart and the color will tell you the PH level. When i put 5 drops of Phenol Red, the solution turned very pink, which is 8.2 for PH level..why is that? I tested the water the same way that i always tested my treated water. But something just doesnt add up. are you suppose to test the PH on RO water? Or do you just put it into the aquarium? The real question is..what am i suppose to do to the RO water???

What is it you are trying to accomplish?

True RO water is very pure and you should get a ph below 6.5. Something's wrong, either with your ph test or your RO machine, Take a sample of your RO water to your local fish dealer and have them test it. Another backup test is to use strips.

The Ph value varies due to the supply of the water, RO water will ALWAYS hold a lower Ph then the tap source.


I use some buffers on my discus tank to establish a safe enviornment and no Ph swings due to low gh and Kh.
On another tank I use straight RO water with NO ill effects and the inhabitants are angels and rams in that tank, the QT's use straight untreated RO water with no ill effects at all.

RO water works quite well if you add the necessary additions to bring up your kh and gh. these will help buffer your tank's ph so it doesnt swing drastically. This can also be helped by adding driftwood, plants and certain substrates. Unfortunately, if you run straight RO water, you're running the risk of decreasing the life of your fish and your plants will have nothing in the water to get nutrients from (unless you use rooted plants with the correct substrate). Fish do need certain elements and minerals to stay alive and do well. You should at least add some minerals to bring up the GH using something like seachem equilibrium. This will give the necessary calcium, magnesium and other minerals that your fish and inverts need to live long, healthy lives. Something easier to do is to do a 50/50 mix of RO and tap water.

You're PH may be so high due to a large amount of CO2 in the water. Try testing it after about an hour of sitting around in a bucket. It should be lower like whiteglovesaquatics stated.

High amounts of CO2 will lower the ph, not raise it. Letting water sit for a period will gas out the CO2, causing a corresponding rise in ph.

Thanks for the correction. Must have been having mixed up thought.

You can also use your RO reject/waste as a mineral balancer, or tap...Keeping a stable tds is a bit more important than the ph value..Gary