I took a 5 gal bucket and made marks every half gallon, I got these little pump off of eBay for 10.00, it’s rate at like 6 gals a minute but actually more like 1 or 2 and adjustable, a length of tubing from Home Depot, I just slide a pc of 1/2” pvc over the end to make it ridged, easier to reach in the back
I just Purchased this water pump, got it to empty overflow when flushing. Used it yesterday to feed. Worked fantastic. Pumped in one gal of feed, let sit 10 minutes than one more gal and it produced 1/4 gal of runoff. Easy peasy
I also use a battery operated syphon.