One bit of information that might be useful to you all is how much filtration media is required per pound of fish. I have more of an aquaculture background than an aquaponic back ground.

A good average per pound of fish is 20-30 square feet of surface area for bacteria to colonize on. I will use 25 ft2 for this example. Different filter medias have different ratios. You can look these up if you are building a bio filter, and size it correctly.

A 3 foot by 6 foot grow bed that is 12 inches deep filled with pea gravel( That seems to be what most people use) has a capacity of 18 cubic feet. One cubic foot of pea gravel has roughly 96 ft2 of surface area. This grow bed has roughly 1728 square feet of surface area. If we divide this by 25 we get roughly 69 pounds of fish. This number can vary depending on flow rate, temperature, and of course root mass from the plants themselves. But this is a great starting point. If you want to be conservative you might down size, and call it 50 pounds. So if you chose to grow tilapia, and your target harvest weight was 1.5 pounds, you should be able to conservatively stock 33 tilapia per 3x6 growbed. Once you have this formula, you should be able to adapt it to other medias, although I have not been able to find the specific surface area of Hydroton so far.
I hope this might be helpful for members who are trying to decide how many fish to actually stock. If anyone comes across specific surface areas of various medias, please post them here.
Here is a link with some info on specific surface areas of commonly used filter medias. http://www.russellwatergardens.com/Styl ... diassa.php Also if you purchase media from somewhere like Aquatic Ecosystems, they usually list the specific surface area, and you can use this formula to size your filter to your desired stocking rate, or your ultimate harvest goal.
Good luck guys! This sure is a lot of fun isn't it?