Replace them when they start to fall apart. For regular maintenance you should just rinse them in a bucket of old tank water that you remove during water changes. Don't rinse them under tap water.
If you do need to replace them, only do one at a time (it sounds like you have more than one). Replace one and then wait a few weeks before replacing the other.
Hope that helps.
If you do need to replace them, only do one at a time (it sounds like you have more than one). Replace one and then wait a few weeks before replacing the other.
Hope that helps.