Backyard Aquaponics
A place to discuss aquaponics

From this thread:
viewtopic.php?f=12&t=6646
which has a lot of really interesting discussions (it's definitely worth reading in full) the generally accepted maximum stocking density is 3 kilos of fish = 100 liters of wet media = 50-100 liters of water in your fish tank. (1 lb of fish = 5 gallons of wet media = 2.5-5 gallons of water in your fish tank.) Which is a little different to what ebeurle posted, but this is all a very flexible thing and the limiting factor here is your growbed. Also, it's usually a good idea for beginners or people with new systems to start with about half that amount of fish so you've got more leeway for making mistakes; you can always put more in later!

Plate size for tilapia seems to be 1-2 pounds (call it 1 1/2); channel catfish get a lot bigger but it looks like 2-7 pounds is a fairly normal range (so let's say 5). I am completely pants at working out gallons and so on, so I did some converting and worked out that you could have around 11 kilos of (grown) fish supporting that growbed, which according to Google's handy converter is just over 24 lb. So that's around 16 tilapia, or 5 catfish, and once you're confident that everything's going well and you know what you're doing you could increase to 32 tilapia / 10 catfish.

Your stocking capacity is based on filtration capacity,you want 25 litres of wet media per full size fish,I personally wouldn't stock by water capacity.
The extra fish will help towards your cuc problems but you need time for the solids to accumulate in the bed and to break down to release nutrients back into the water.Pics would help towards a solution or cause of your problem.

I got 15 talipia and 1 small channel cat, the talipa range in side from a finger to a brick, I will check them when I get home and see how they are doing hopefully nobody died.