PH keeps dropping

[HOCATKA]

My PH was very stable for a while, but recently it started to drop and doesn't go back to what it used to be. When I change water it comes back to about 6.0 but then goes back to 5.5. Because of that my CO2 regulation system doesn't work properly. I have very hard water so I use 100% RO and then add "RO Right", plus ferts for plants. Any suggestions? Thank you!

[poconogal]

What is your KH? Since you use 100% RO, probably one way to help with this would be to use less RO and add some tap back to your water. Perhaps a 75% RO to 25% tap mix. You'd have to play with the amounts to see what works. Since I don't do planted, I'm not sure of the effect of plant ferts on KH, but perhaps reducing the ferts somewhat would also help? I know excess fish waste can acidify your water, so maybe ferts would as well. But I'll leave that for the planted gurus.

[Alight]

Change much more water. Do a 70% change twice in less than a day and you should be back on track.

You have established a low pH buffering system, probably with nitric acid as the base, by not changing enough water, often enough.

The other option is bicarbonate to buffer the water up, but I do not recommend this, as your low pH is telling you that you have signficiant pollutants in your water that would be better removed.