If things go as expected, we will have 250 young european catfish swimming in our system around Christmas. They are still small, and I think that one, maybe two tanks will be enough space in the beginning. But they are expected to grow, and reach a serious size in let's say a year. That means that we need more tanks in the near future, and if we look at the rules of thumb for aquaponics, we are supposed to end with something like 15 fish per tank..
In the ideal situation, where none of the fish will die along the road, this would mean 15 tanks for these guys alone, to be realised within one year