These blocks are made from the same ceramic balls as the media used in the sewage treatment industry so they are designed to remove waste.
Highly porous with massive surface area for the colonisation of useful bacteria in marine aquraiums, won't easily break down.
Usually takes 4-8 weeks to establish bio load. After that remove once a month when doing a water change. Rinse them off in the water you're going to discard & siphon the sump where they were to remove any detritus that has settled there.
Each block is 10cm X 10cm X 2.5cm