I guess best is to ask the experts directly or look for some related threads:
Backyardaquaponic is the best forum for Aquaponics I know of.

Here is a link that might be useful: backyardaquaponics

Hi Chris,

In a media based aquaponics system you want to strive for about 6 gallons of fish-water to every cubic ft of grow bed (and since grow beds should be 12' deep the calculation is pretty easy ;-). Then the rule of thumb is 1 lb of fish (or about 2 small to medium fish or one big guy) to every 3 - 5 gallons of water, depending on how mature your system (re: your biofilter) is. Hope this help!

Here is a link that might be useful: The Aquaponic Gardening Blog

thank you Lucas. Thank you slywoman. That was very helpful. I'm gonna use gold fish. I wonder how many of those in a pound. My grow bed doesn't fit the bio filter requirements that you gave me, and I don't know if the pump is big enough, but I'm gonna see if I can cycle it.

So why do I need a 12" deep bed? Is that for root support?

and what is the 1:1 ratio for? Does that mean I need that much media to use as a bio filter? I'm using 40l of expanded clay, but its the wrong kind so it will always float. So I decided to fill the bed with enough water that it doesn't start to float and just let water flow out...this will kinda be like a circulating DWC.....maybe 15l are will be circulating 25 or 27 gallons of water