KH crashing after water changes? I'm scared to do another

[FreshwaterFan]

Did a 30% water change on my 20 long yesterday and now my pH went from like 7.6 to 6.8 overnight. My betta is hanging at the top and I'm freaking out a little. Tested with API liquid kit, ammonia 0, nitrite 0, nitrate maybe 15. GH reads around 6 drops, KH reads 2 drops. Is that low? I keep reading that low KH means pH crashes but I don't really get why.

I use tap water with Prime. Let it sit overnight with an airstone. No driftwood except one small piece. I do have some Fluval stratum in there from when I tried plants, could that be pulling the KH down? Or is it the tap water changing seasonally? I'm on city water and sometimes it smells like pool.

Should I add crushed coral? I have a bag of it somewhere. Or do I need the Seachem buffer? I don't want to chase pH and make it worse. But I also don't want to wake up to a dead fish. Anyone dealt with this? How fast is too fast for pH to move?

[rk_designs]

KH of 2 drops is around 2 dKH which is on the low side but not apocalyptic. The crash zone is more like 0-1. That said, a 0.8 swing overnight is enough to stress a betta, so your instinct to worry isn't wrong.

The Stratum is almost certainly part of it. That stuff is an active substrate, it strips carbonate hardness out of the water to buffer itself down toward the 6s. I ran it in a 10 gallon and watched my KH go from 4 to basically nothing in about three weeks. Pulled it and the tank stabilized. If you've got a decent amount of it still in there, that's your culprit more than the tap.

City water with a pool smell is chloramine, not chlorine, and that's a whole other headache. Prime handles it, but letting it sit with an airstone overnight does nothing for chloramine, only chlorine offgasses. So if your utility switched over seasonally that could explain a shift too. Ask your water department for the current report, they publish it.

Crushed coral, yes, but put it in a mesh bag in the filter, not dumped in the tank. A small handful is plenty for a 20 long. Go slow. Do NOT reach for the Seachem pH buffer, that's the classic newbie trap and it'll give you a yo-yo pH that's worse than a stable 6.8. Also side note, I tried to order a bottle of that stuff last month and two vendors wouldn't ship to my state, which was annoying, so there's that.

How's the betta actually behaving besides hanging at the top? Gasping, clamped fins, or just parked up there? If it's just hanging near the surface it might be oxygen, not pH. What's your tap pH straight out of the tap vs after 24 hours? That gap tells you a lot.

If you want the full rundown on params, there's a complete betta care guide on here that covers this stuff.