Hello everyone. This is a question I can't quite figure out:

I am plumbing a greenhouse. We have two 1000 gal reservoirs, filled with RO water. Coming out the bottom of each reservoir is 2" black poly, with a T and a reducer to 1 1/4, which connects to a 3/4 hp shallow well pump with a 13-14 gpm capacity. We are running a 1" black poly, maybe about 80' with 5 or 6 90 degree turns, and currently I have a shut off valve, with the final discharge into a different reservoir where plant food is added. The shutoff and pipe discharge is about 7 or 8' above the shallow well pump back at origin. Our problem is that when the shutoff is used, after filling the secondary reservoir, the pressure drops in the black poly pipe, and the shallow well pump does not automatically shut off, due to pressure increase.

Am I right in guessing that I need a check valve on the intake side of the well pump, in order to keep pressure in the supply side of the pump (and the pump itself)? Or, Do I need to add a pressure tank on the supply side, which would include a check valve in the Tee package....?

(My understanding of a pressure tank is so that you don't short cycle the pump for a small drawdown of water, like brushing your teeth... thus sizing the tank to allow for various fixtures to be used without lowering pressure and turning on pump... but we just need water for to fill the reservoir at a pretty significant volume, so do not have a short cycle issue...).