In Aquaponics, Flood and Drain gravel beds make really good filters, not only for bio-filtration but also for solids filtration.
However, the water going into my water fall tank was direct from the pump out of the in ground fish tank. I wanted to add some filtration before the water went into the waterfall tank.
I had previously tried a “swirl” filter but my flow through it was too fast for most of the solids to actually settle out and I didn’t like having to dump the solids out of it every other day. So I tried making another type of filter where the water would simply flow through a bin full of gravel and flow into the waterfall tank. That idea didn’t work for long though and was clogged up and flowing over the top withing a few days.
Next I hooked up an upflow gravel type filter. In this the water is pumped into the bottom of the bin and flows up through the gravel before flowing out of the bin. This worked far better for cleaning up the solids in the water fall tank but didn’t really provide much growing space for the filtration.
While the up flow filter did seem to work for the Aquaponics system I would prefer to use a regular flood and drain grow bed for most of my filtration.
Any filtration where the water splashes in at the top risks clogging and overflowing as the media slimes up. I have seen this on grow beds with constant inflow in a single location. Timed flow to the beds seems to alleviate this trouble. Upflow filters don’t have the splash and overflow problem but they don’t have much aeration into the media.