toilet deodorizers

Question

There are these deodorizers that look like hockey pucks that you can drop into the back of your toilet, and essentially each time you flush in cleans the toilet with the normal water that goes through the cycle. Question is: these pucks last for three months and they work like a bar of soap, in that they have layers and get smaller as time goes on... just like bar soap is prohibited on shabbat, should these pucks also be? thanks