The bioballs are to help set up nitrifying bacteria to convert ammonia eventually into nitrates. They're not designed to be mechanical filter, which would remove all the junk from your water. Sponges are often in biocubes for that reason. I removed the bioballs from my nano because they can become 'nitrate factories' when they start to get dirty. I have live rock in my tank which serves the same purpose and also removes the nitrates from the tank by converting it to nitrogen gas. Bioballs can't do that. I now have Seachem purigen, chemipure carbon and Seachem denitrate instead of bioballs. If you plan on keeping them, rinse them off once a week when you change the water in the dirty tank water you've just removed. That will keep them clean. Lots of people don't use bioballs anymore because live rock is so much more effective.