i have a 18g tub for my top off, actually have 2 of them one in use and the other filled with ro\di water, day before one in use is low i mix kalk in new container, let sit 24 hr's then switch tubs out, each time ato kicks on it is dosing kalk water. does cause a slight spike in ph for about 30 sec usually goes from 8.4 to 8.5 or 8.6 way my ato unit is set up the level doesnt drop much so it only add a little top off when it comes on and use a aqualift pump for ato so not much water goes at once, i do have the secondary float about a 1\2 higher then one that turns ato on and off, this way if it gets stuck the secondary switch will shut system down { learnt after crashing last system cause a snail decided to hang on flaot switch and wasnt a back up to turn it off