I forgot to mention that those drop checkers are really not that accurate. Especially if you are making micro bubbles. The tiny bubbles get lodged inside the drop checker & give you lower readings than you think you have. They are general guides only & can lag hours behind what the tank is in real time.
As far as your tank getting lower as the day goes & needing HOB filters to help off gas the excess CO2 , that would mean if you are running a canister that you need to adjust the canister so it moves the surface more but not braking it!
I have miss adjusted my filter many times , you just have to be more diligent in your adjustments & play around with it some.
I like running outflow pipes more than spray bars but sometimes both is better running 2 separate filters.
I have used HOB filters as stated to out gas CO2 but found they waste a lot of gas. I know people will say the gas is cheap but a properly positioned Canister will beat any HOB filter from what I can see. What do I mean by beat? Better bio filtration , proper surface agitation , more bio media capacity, more flow options , better filtration & less bi pass, less surface scum, inline CO2 reactors, generally moves more water, out gasses less CO2 if properly positioned, less stuff hanging on the tank . The list is too long to list.