If I plan to add any fluids from the top, I make sure to turn off my incoming water/nutrients tubing. I have valves all over my setup. I'd like to be able to turn off individual AutoPots as well as having a master shutoff valve. I even have two valves surrounding an inline filter I added.
Sometimes I even go so far as to drain my AquaValve reservoirs. That way, anything I put up on top doesn't push the fluids in the AquaValve chamber to the point of overflowing.
The pump I bought is a simple type like you would use to empty a motorcycle fuel tank, etc. I'll include a picture of the one I bought from Amazon. I can't post the link, but you can search for it.
An example of why I would drain the valve compartments on the AutoPots is like when I want to put some Recharge into the Coco (soil). That water is going to go down and exceed the capacity of the AquaValve chamber (if left at its natural state).
Another idea, when I think ahead, I'll just shut off the master valve from the main reservoir at night when I go to bed and in the morning check to see if the pot and medium has sucked up what was in there last night. Usually that fluid is gone or mostly gone.
Then slowly add in waterever you want to pour on top and watch to make sure you don't spill over. If I keep it below the natural waterline I just turn on the main valve and let the valves come up to their natural limit. That way whatever you poured in and passed through still get back up in the soil.
Edited to Add: I cut the hose on the inlet (suck) side to about 6 - 8 inches and run the outlet hose to a bucket or anything else that will hold the water. If that is not too nasty, I pour it through a filter and then EC test and pH test it. If nothing needs to be changed, I just dump that back in to the main reservoir.