Aragonite to buffer water

Yup, aragonite is a tad more effective than crushed coral as it dissolves more easy, but it is not a huge difference (I used it when I had low KH at the office - I don't at home). Start with a couple of tablespoons and monitor from there.

I don't mean to change the subject at all, but if you add aragonite in your tank, could you still use regular (untampered) tap water for replacement even if it doesn't have the same KH or pH? Wouldn't the difference cause stress towards fish?

When you have KH ~3 and want it 4-6, the TDS difference is small enough to be trivial and non-shocking. What you are doing is slowly bringing the KH up a bit, rather than having it decrease between water partials. Fish do not read pH, they read osmolarity - which we can check most easily by TDS, but most hobbyists do not bother.

If you were working with soft GH and low KH water or with RO water and wanted to bring it up several-fold, as you might do for F1 Rift Lake fish, then you would premix the appropriate salts/minerals just as you premix marine mix for BW or SW. The degree of change is the difference.

When I do a 50% water change my KH drops about 1 dKH from where it was before I started the change.

I'm at 1-2 out of the tap and it's buffered to 4-6 with aragonite/coral and no CO2. Meaning that 4-6 is the span of three of my tanks in a morning reading before the CO2 kicks in. Once the CO2 kicks it, it usually goes up 1 dKH, so my tanks sit at 5-7 dKH during the day.

Roan