I recently changed mine from pumpout (dump through) to cassette. 1 of the problems I had was I was getting less & less use out of it each time, to the point where it would only last a week, & certainly didnt look like it had been emptied even though nothing more would come out. I assumed the pipe through which you pump out was probably rotting in the tank & didnt reach as far down as it used to. Anyway once the tank was removed it wasnt the pipe (which was plastic & hadnt rotted), it turns out the damn tank was over half way full with dried up loo roll. I did my research & opted for pumpout when I first bought my boat. Everything I read said NOT to put chemicals in there (think Elsan Blue), the idea being biological action action would break down the contents into slurry. For this stuff to broken down into slurry, it needs to be covered by liquid, hence the poo stick that people have mentioned, else you get a pile of poo & paper that isnt covered by liquid. Far better to shove it out of the way rather than fill it with loads of water. (I believe I broke mine by not peeing in it enough) The idea of Elsan blue containing Formaldehyde is that it kills all biological action dead. (& it's perfumed). It makes no claims to do anything else. So dont use it in your tank. Thetfords version of the stuff is called Aquakem blue, & they say they havent used formaldehyde since 2003 (although I'm sure I read somewhere it has 10% formaldehyde but I cant find it again). This Aquakem blue also claims to help liquify the waste. There are other products as well, & as other have said Baby Bio, Biological washing powder, etc. The point is you need to get that stuff liquified. That was the problem with my toilet, it wasnt liquifying, & over time the paper built up & stayed in the tank.
As an aside, once I'd changed to a cassette toilet & went looking for the various 'blue' products, I came across loads of posts where people did use Elsan blue or other products containing formaldehyde in their black tanks.