New 10 gallon Walstad setup - Ph rising?

[KaryPerry]

I reset up a 10 gallon tank almost 2 weeks ago attempting the Walstad method. I used existing sand and driftwood over Miracle Grow Organic potting mix. I added some granite rocks I had in another tank. It has twice the plants it did before. My Ph was always 6.8 - 7.0 in this tank and the other the granite came from.

When I first set it up my Ph was 6.4. I figured it was lower than normal from the soil. The water was clear that night, but the next morning a bacterial bloom had begun. I waited it out for a week. By the end if it the water went from milky to dark tea stained. The dark water color is pretty weird. I don't think it is from the soil, as it would have showed up in the beginning. I tested the water again. Everything was at zero (ammonia, nitrites, and nitrates) except the Ph climbed to 7.2.

I did a water change, added dechlorinator, and let it sit for 3 more days. Water is clear now. I was going to add my frog, betta, and snail back tonight, but the Ph climbed again to 7.4. My tap water is neutral. My dwarf hairgrass is dying. The other plants seem okay.

I am scared to put my critters in the tank with an unstable Ph. They are currently in a 5 gallon tank stripped of plants.

Any ideas what could be causing this? The only new thing added is the soil. I will be setting up a 29 gallon tomorrow using this same method, but with river rock decorations instead of granite. I am wondering if I did something wrong.

[John K]

I've experienced this myself when using MGOC as a substrate. It seemed that something in the potting mix raised my KH, which raises pH. As long as it doesn't keep going up, I wouldn't worry about it. Fish don't care about pH, they care about general hardness (GH).