The water in Lake Erie is a LOT cleaner now than it was in 1980, Mitch...do you think you'd still need a dedicated flush water tank? Even if you do, or now sail in sea water, there's a better way to flush with fresh water SAFELY than installing a dedicated tank...it's an easy job on all boats that have the toilet and the head sink on the same side of the keel:
Sink drain thru-hulls are below the waterline on almost all sailboats. So re-route the toilet intake hose to tee or wye it into the sink drain line as close to the seacock as possible because the connection must be below waterline to work.
This will allow you to flush normally with sea water. After you’ve closed the sink drain seacock in preparation to close up the boat (you do close all seacocks before leaving the boat to sit??), fill the sink with clean fresh water and flush the toilet. Because the seacock is closed, the toilet will draw the water out of the sink, rinsing the sea water out of the entire system—intake line, pump, channel in the rim of the bowl and the discharge line,(Water poured into the bowl only rinses out the toilet discharge line). If your toilet is electric, be careful not to let it run dry…doing so can burn out the intake impeller. Or you can keep the sink drain seacock closed except when it's needed to drain the sink and flush with fresh water down the sink all the time...your choice.
It may also be necessary to keep the sink plugged except when in use, with a rubber sink plug or by installing a conveniently located shut-off valve in the drain hose. Otherwise the toilet may pull air through the sink when you try to flush, preventing the pump from priming.
I first learned about this solution to sea water intake odor from a Tartan owner in the mid '90s. At that time Tartan and some other boat builders plumbed their boats to use the sink drain thru-hull for the toilet intake too, 'cuz it eliminated one hole in the boat (toilet intake), and also saved 'em the cost of another seacock. I thought it was such a great idea that I've been recommending it ever since.
--Peggie