Fish farming is the fastest growing industry world wide. Already just under 70% of all fish consumed comes from farms and it is estimated that by 2030 this will be over 90%.
Here is an example of a Fish Farm system that has the best equipment available (of course, we can go smaller or larger by using various configurations).
A cage sized 120 meters in circumference x 10 meters deep will hold a fish harvest of about 345 tons with a medium stocking density of 30kg/m3. With fish that grow like salmon you will want to harvest any weight in excess of 300 tons every day. If their weight is growing at 10% per week, you would want to harvest about 35 tons from each cage; if you had ten cages that would be a harvest of 350 tons per week. From this example you can see how quickly a fish farm can be self funding from the first year, once you have harvestable fish.
If well managed it is possible in the first 40+ weeks to obtain a FCR (food conversion rate) of 1-1 (basically creating protein at that rate). Salmon can yield a yearly average FCR of 5-1; tuna, 23-1: this means that for every 23 kilograms fed you get 1 kilogram of growth weight.
For a complete 14-cage system including mooring grid and nets, assuming the best quality, with 14 double collar 120 meter circular cages, a budget price would be US$2,176,472.00 or $155,463.00 per unit. From this you can estimate the number of cages that you want and their size (80, 100 or 120 meters in circumference), keeping in mind that fish farms work on economy of scale.