Originally Posted by MyRainbowBettaFish
You should NEVER EVER replace your cartridges unless they are falling apart, but then you have to stuff the old media into the new media to keep your cycle. If you throw it out every month, it will just destroy your cycle.
I had to replace my cartridge after about 8 weeks because it was so gunky water was flowing over it and out of the overflow spout thing, then agitating the water too much and Fishie started biting his tail.
Someone on another forum advised me to stack it on top of a new one in the filter for a few weeks, then remove the old one and stack the newer one on top of another new cartridge so that next time that happens I'll only be removing half of the BB when I have to replace the cartridge. Luckily, it can juuuust fit the two of them on top of each other in the filter under the spout thing. It also helps baffle it for when I put Fishie back in. I also am still letting the old cartridge float in my tank for a little while longer to make sure the new cartridges get established.