There are also controllers to monitor and automatically adjust reservoir pH. I use these at work for a flow through wastewater treatment system, and at my previous job for industrial fish tanks. Yes there are industrial fish tanks, it's a thing, and I have swum in them (with the fish, no less, eww, no wetsuit, no scuba gear, absolutely disgusting) more times than I'd like to remember. Seriously it's the stuff of nightmares and definitely violates OSHA confined space entry regs. Worst job I've ever had. And I've had some doozies.
The basic principle is you've got a probe in the tank, which needs to be cleaned every couple days, hooked to a controller (preferably also to an alarm) where you set high/low parameters. Then the controller outputs to a dosing pump which controls a feed from a chemical reservoir.
Since you probably already know which way you'll drift, you choose either pH up or pH down. A two pump system is much more costly and a bigger PITA to operate.
Let's say your res drifts down over time. You'd choose pH up, and set your parameters to dose when the pH falls below say, 6.4, and to shut off at 6.8 (this is just an example). When the probe input to the controller falls below setpoint, the controller calls for some caustic and activates the pump. Usually this would be a peristaltic pump which is quiet and low flow. It pumps in some solution and shuts off when it hits the setpoint.
I have seen these systems fail catastrophically resulting in Very Bad Things. They need to be cleaned and inspected daily, and you need some type of constant mixing in the res to make them accurate.
For a home grower it's more expense and headache than it's worth. You'd see a setup like this in a larger commercial operation with a daily inspection log to confirm someone has been wiping volume off the probe and calibrating the controller.
