Alkalinity is the buffer your system requires and is depleted by nitrification. Unless you are also performing denitrification than it will go down.
Tropic Marin has a simple KH test kit. Not the pro. Works on the same principle as API. Add one drop until it changes color. Each drop represents 1 dkh. Super simple. Quick. Can be adjusted to provide 0.5 dkh but I don’t bother with that precession. I aim for 9 knowing it can be off 2 dkh in either direction and all is fine.
Tropic Marin has a simple KH test kit. Not the pro. Works on the same principle as API. Add one drop until it changes color. Each drop represents 1 dkh. Super simple. Quick. Can be adjusted to provide 0.5 dkh but I don’t bother with that precession. I aim for 9 knowing it can be off 2 dkh in either direction and all is fine.