My suggestion is that if you are raising Tilapia and using Hydroton as your bio-filter media, then you will want at least three gallons of bio-filter media for every one pound of fish at maximum grow out.

If you plan on using the bio-filter just for filtering then place the filter media in a vertical container, feed air (using aerators and lots of air pump volume) and the waste water from the fish tank into the bottom the this vertical filter. That way the air and water will travel vertically up through the filter and in the process help remove the fish waste solids, which are heavier than the water, and will remain near the filter tank's bottom where they will be broken down (mineralized). The water flowing out from the filter tank's top will be relatively clean. If the filter tank is large enough, then the solids will have time to break down before they over accumulate.

Be sure to move enough water through the filter so you will be removing the solids from the fish tank. The water can then be returned to the fish tank and some of it can be used to supply your grow beds whether or not they are media filled. You can also use a separate pump of appropriate size to move water to the growing area.

One key to making this work is ample dissolved oxygen in the filter for the bacteria to convert all the waste to nutrients. The water flow up through the vertical filter will be relatively slow, thereby allowing the solids to remain suspended and not move all the way to the top where the clean water will overflow. This requires a large enough diameter of the filter, relative to the pump flow, to achieve this slow upward moving condition. Too slow of upward movement will allow the solids to over accumulate near the bottom of the filter. Too much upward flow will allow the solids to make it to the top; but if the filter is large enough, this is not likely.

Oliver