The B219 holds 21 gallons - not 20, and on a boat that missing-then-found gallon tells you something about how Ronco works. The shape came first, drawn to fill a specific class of hull cavity all the way to its corners, and the capacity is simply what that shape holds. It is the opposite of forcing a round-number tank into a space that wanted something else, and it is why the drawing below deserves a careful look against your own compartment.
The molding is marine fresh water stock: rotomolded FDA-compliant polyethylene, potable-rated, one seamless piece with nothing to corrode. It ships at 89 lb, blank. Locate the pickup for how the boat sits on her lines, not for how the tank sits on the bench.
In service, plan structure for about 175 lb of water at 8.34 lb per gallon. Flat bearers, straps to something that will not move, fittings left reachable for the clamps you will service later. Standard practice from a shop - Tustin, California, since 1974 - that has watched its tanks outlast the boats around them.
Neighboring capacities and very different outlines are in the boat tank category; the full molding list is on the Ronco Plastics hub. One Source Plastics is an authorized Ronco Plastics dealer.
Fittings
This tank has a molded shape (see the drawing), so it is sold as a blank tank with no fittings installed. Add the Ronco 3-inch uniseal (F3UNISEAL), a loose pipe-to-tank seal you install yourself, if you need a sealed pipe connection.
Technical drawing: download the Ronco Plastics B219 dimensional drawing - print it to plan your install. Sold blank, with no fittings installed.
