I know this post is old but I just wanted to say I'm doing really well with sand in my little aquaponics system over a bathtub with fish. I don't use a pre-filter. I started with 4 standard (in Australia) seed planting trays with a chux cloth in the bottom to stop the about 50mm deep of sand falling through and two containers making up the same size area as the sand beds containing about 300mm deep of scoria. a pump takes the water out of the bathtub and distributes it through 32 little 3mm fittings to evenly disperse the water across the media in a continuous flow. there are 10 juvenile Jade perch and some feeder fish in the bathtub plus a catfish to keep the algae at bay. The sand side outperformed the scoria side by leaps and bounds. The plants in sand were a lot more productive and when I dismantled the system and compared the root systems it was obvious why. The plants love the added support and probably some sort of mineral supplement they get from the sand and their root systems went nuts. I've just re-jigged the system so it's all sand with fresh plants in all of it. The plan is to change one seed tray every fortnight empty out the old plants, wash the sand and put the water from the wash on the garden. For my bigger system I'm using the triple sump system to remove all solids before it reaches the media bed. I'm going to do a compare trial of sand versus scoria in that one too. Scoria already outperformed clay balls in a previous test. Hopefully the triple sump will negate the need to wash the sand as this will be impractical in a larger grow bed. This comment peaked my interest... " The removed solids can (and should) be mineralised so that the nutrients can be put back into the system" Is anyone able to explain how that works?