volume really depends on the number/density of fish you have. for instance, if you have 20 juvenile tilapia (common with aquaponics) you don't need as large of a growbed as you would with 20 adults. There are volume calculators made for just this purpose on several aquaponics sites. In aquaponics plant density is increased because you don't need soil.. this means that you can have a larger number of plants in a smaller area.. I am currently running a 6 square foot growbed with houseplants and 4 juvenile central american cichlids. I could increase my density quite a lot if i were to go with tilapia or koi or even bluegill as they do well and grow well in very dense populations. If i were to do that i would increase my growbed volume.. probably close to 12 to 16 square feet (8 inches deep with expanded clay media) and plant some tomatoes, lettuce, peppers, cucumbers, etc.. right now, my house plants do well enough to deal with my juvies.