5 gallons is too small, but 6 gallons isn't??
The difference is only a MAXIMUM of 2 flushes--1 per person--and then only if you're frugal with flush water...certainly not enough to make the difference between spending one more night aboard or inviting even one more person.IMO, unless the tank is only for show and will never be used, 12-15 gallons is about the smallest tank that's worth the money and aggrevation to install it. The difference in cost between the smallest tank and one that size is minimal, the cost of the hose and fittings is the same...and so is the amount of time, effort and aggrevation.