Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have heard the certain RO membranes will give you this odd pH result. Here is what I would do:
Run some RO water into a glass. Test the pH right away, the moment the RO is exposed to air, then let it sit overnight. Test it at 24 hours and at 48 hours.
Similarly, take a handful of your substrate and put that in a glass of water (aged water, if you have some- has been exposed to the air for a day or two). Again, test the pH right away, and at 24 hours and 48 hours. You might have to run this one out a few more days. If you have any rocks or any other decor in the tank you can do the same, but I would start with the substrate and the water itself.
Goal: See if the pH change you are seeing is something to do with the RO water itself, or if there is something in the tank (for example, the substrate) that is doing this.
For fish the most important parameter is the mineral level of the water, not the pH. I do keep hearing about the pH when people are keeping shrimp, though, so maybe that is more important for them.
I hope by cycling you mean you are adding ammonia. That is how you feed the bacteria and keep them growing.
The bacteria will grow best with more minerals in the water. They use the carbon from carbonates, and need some phosphorus at least. Perhaps more things.
While you are raising bacteria I would make the water optimum for them (raise the GH and KH to at least 3 German degrees of hardness and add some plant fertilizer that supplies phosphate). You can change the water right before you add shrimp, and the bacteria will be OK, though they will not grow very much if the water does not have the minerals they need.