Advice? Bulk head size?

This might help you out. Savko Plastic Pipe & Fittings

 

Bulk reef supply will have a pipe size vs hole size chart if you go to their bulkheads. FYI those holes are drilled for a standard abs bulkhead. A schedule 80 such as those at farm fleet stores won't work. But if the hole diameters are roughly 1.5" and 2" then it's most likely 1" and 1.5" pipe sizes. The larger being the drain

 

It's worth the money to buy the hd stuff from brs

 

just an FYI, when the bulk heads come in make sure to debur the flages. Also, hand tighten only. Never tool it tight.

 

You can go to sprayer specialties here locally in Des Moines I have used them before and as long as you know how to install a bulkhead properly you will b fine..go with 2 inch in case you change your mind for more flow/larger pump IMO

 

If the hole is 2" a 2" I.D. pipe won't fit.
Stay safe, go to BRS and check their sizing chart. Hole size to pipe size.