Bio Pellets need to be changed or not ?

[kien]

Like everyone else, I merely top up my pellets every couple of months. Everyone's pellets will deplete at different rates.

Changing out your pellets wholesale would actually be detrimental to your system if your system is relying on the pellets as the soul or primary nitrate&phosphate sink. It usually takes a while for the bio polymers/pellets to develop the bacteria that consumes the pellets, along with nitrates and phosphates. Changing them out would mean you're starting from scratch every single time, and that usually isn't a good thing.