I have no experience with marine heads. My former boat, a Hunter 25, was equipped witha porta potty. My new ('73) Pearson 26 is equipped with a marine head and holding tank. Sitting at the base of the head, fitting half way around it is a small holding tank, not hooked up. This small tank is the shape of a C. What I am wondering is whether this is a portable tank, which would give the head the sturdy seat of a marine head but the convenience of a porta-potti (no pump out required for day sailing). Or did this small tank simply come with the marine head when it was purchased, and is just sitting there unused because the large holding tank exists. Should I remove the small tank from the boat and store it in the garage? Or are there quick and easy fittings for usage as a portable "day" tank? How do I find out? Thanks!!! Rick