Back to Front

I've set up my system and added a big grow bed I had used earlier for hydroponics. Now as I say 'back to front' I have to calculate the number of fish I need.  I'm going to check flow to see the full cycle time of three different tanks and then work out the suitable number of fish the system requires to feed these three beds. Fish density is not my main concern as I ca see water throughput is quite high. I need to make sure I get enough fish waste to make it sustainable.

I've got about 180 litres of water, two half full barrels with aggregate and one grow bed 2.4 x 1.2 x 8cm deep.  Will update when I've got this sorted out. Off to by some seeds.