Who Gives A Fish Forums

I'm chiming in with a similar setup - heavily planted tank, light stock - but my pH is actually drifting down instead. My tap water pH is usually between 8.5 and 9, and my water hardness is around 12dGH. Today was a surprise, though - I had to switch to the regular pH test bottle because my reading was off the charts on the high range test liquid. It turns out my pH has dropped to around 7.4, while my tap water is still sitting at 8.5. I've seen this mentioned before in planted tanks, but not often - usually it's the other way around. So, like others have said, I'm guessing it might be the rocks or something else in your tank causing your pH to spike. Looking forward to seeing the results of that rock pH test, by the way.

When plants take in nutrients, they don't use them all equally. I've found this out the hard way in my RO water tank - without a GH booster, my shrimp would be in trouble and my plants wouldn't thrive. These boosters contain calcium and magnesium sulfate, which plants need to grow. They require a lot more calcium and magnesium than sulfur, though. So, when my plants use what they need, the excess sulfate stays in the water, causing the pH to drop. If I don't intervene, the pH can drop below 5.

I suspect something similar is happening in your tank, but in reverse. With excess potassium, the pH would likely increase. Unfortunately, it's impossible to create a fertilizer that avoids this issue. Regular water changes are the solution. If you don't change the water, the excess minerals will build up, pushing the TDS levels too high and harming your tank's inhabitants. Even without fertilizer or plants, fish food can cause the same problem, and some rocks can contribute to it too. Water changes aren't just about controlling nitrogen levels; they also help stabilize pH, GH, KH, and nutrient levels in the water.

@shakinStevens I appreciate your detailed explanations. I'm wondering if dosing a lot of Prime might have contributed to my pH fluctuations. I somewhat hastily converted my planted cycle to a fish-in cycle and have been dosing Prime accordingly. The bottle mentions "hydrosulfate salts," which I'm not entirely familiar with. I'm curious about how these salts break down - could the excess sulfur be causing my pH to drop?