Mystery of decreasing pH after water changing with higher pH water?
I'm in the 4th week of a new 50G. It is 80 percent planted at this point and most everything has doubled in size. There are maybe 30 endlers of various size, from fry up. And there is an abundance of shrimp. It's obviously still cycling although it has been days since I detected any ammonia. I have not yet detected nitrites.
My nominal pH, the pH when CO2 is not being injected, has dropped from a high of 7.6 and seemingly each time I've done a water change. I just did a 50% change last night and nominal pH dropped from 7.2 to 7.0. But here is the rub, my tap water is pH 7.6, so why would this be happening? Here are the water parameters:
Out of the tap: pH 7.6, 5dKH, 11dGH
Tank prior to 50% water change: pH 7.2, 5dKH, 14dGH
Tank after 50% water change: pH 7.0, 5dKH, 12dGH
I have an abundance of Seiyru rock which will creep my KH and GH up over time and seems to stabilize around 7dKH and 16dGH if left without water changes. I would assume this rock would RAISE my pH however. I also have one largish piece of Manzanita. It was sandblasted so it is not red and has not generated any tannins that I have noticed.
I have been targeting a pH drop to 6.5 for CO2 enrichment. One theory is that the CO2 is not completely out gassing during time it is turned off and this is causing my pH to appear lower even when CO2 is off for hours. I have reduced current in tank recently because it was blowing fish and shrimp all over the place, so maybe? But I run an airstone every minute that CO2 is not on so not sure this theory holds up. And I have two 200GPH canisters even though I added foam to blunt the current.
Another theory is that this is part of the cycling, in particular when nitrites are produced as those are apparently acidic. But I have never detected nitrites. Ammonia was detected at <.25ppm several times although ammonia has been absent for a few days now. Nitrates range near 0ppm to 5ppm (I have been dosing with Seachem Nitrogen when nitrates are low). Besides water changes, I have managed ammonia with full tank dose of Seachem Prime 2x/day, Seachem Stability and Pristine daily.
A third theory is that it may be due to dosing with Seachem Potassium, which I just learned Potash may be an acidic source. I do notice a temporary pH drop when adding to the tank as I pour it in the tank near the Milwaukee pH controller probe. But that pH drop seems temporary and it comes back up. I have been dosing K at the ~ 2ppm / day level (the dose on the bottle instructions).
Not sure this has anything to do with it, I am using Seachem Flourish tabs and due to that and fish load, I dose lean except for Fe and K. Substrate is inert Seachem Flourite Black Sand (with Flourish tabs).
Any ideas on what can cause this mystery pH drop?