The tank is probably a holding tank.....I had one under my WC, it may have an overflow, but by the sounds of it it doesn't have any sort of filter system.....It'll be diffcult to identify what you've got unless you've any history on what was installed, but I reckon you'll maybe have to install a fosse (assuming that the exissting one is not to the minimum size and a sand filter of some sort. Start by getting a couple of devis, hopefully in the process you may learn some more about your system. Bon chance. M.