Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you can get a TDS meter, then check your RO water with that. When the equipment is new the TDS meter ought to read under 10 ppm, probably 1-5 ppm.
As the membrane ages it lets more 'stuff' through, and the TDS will start rising.
You make the call, but when the membrane is letting more stuff through that is a suggestion that it is time to get a new membrane.
RO filters do not work so well when there is a lot of calcium in the water. Somehow this clogs the membrane and is hard to get rid of.
If your water is high in calcium you might look into a water softener to get rid of the calcium, first. Usually this would be a sodium exchange water softener. RO filters can handle sodium better (they last longer).