I used it exclusively in my last grow, 5 gallon res with blumat drippers. I never had to dump the res, as it would only last about 5-7 days. A quick wash and refill was all I had to do.
There’s a post i’ll dig up real quick…
AH! Here it is
Stable in the res (Holds PH well, and doesn’t smell really bad at res change time), easy to dissolve, and cheap.
I would highly recommend it. To the just topping off? I’d say if you add a bit of H202 (like 1/4 tsp) every “top off” with monthly washes you’d be fine. Just don’t do like me an contaminate your own res because you were being lazy and didn’t wash a transfer container out fully haha.