You might also consider installing a 'bac degraisseur', if any water from wash basin, kitchen sink, washing machine or the likes are connected (should not be but too often are) to the fosse then that compounds your problems. That will need a simple clean out each month or so to remove fatty deposits that, if they go into the fosse, damage the bio function and also take the overflow from the fosse. If done properly, after going a fairly short distance through a filter bed in the ground, the water should go into a stream more or less clean. You will need to carefully check all of your outflow pipes, some people are really devious installing illegal inflows to fosses. Do as Ben says, using Tarax, Eparcyl or whichever you choose and chuck a dose in the bac every time you clean it, about once a year a single dose of Tarax or similar into the the tank rather than the receptor. The dead animal trick is not clever since whilst it seems to work according to folklore, the potential for a major overflow block and then big trouble is on the way. It sounds like your fosse is (no longer) regulation, like very many houses, so do not let anything happen that is going to cause you big problems and an even larger bill. Get a vidangeur in quick. Make sure you have the bill and certificate for officialdom and insurance, if anything happens after the clean out and make sure that the company refill it - if not the smell for several months alone may make you wish to move a couple of hundred km away!