The Biology: What you need to know to be an aquafarmer

The critical variables you can manipulate are stocking density, dissolved oxygen, and protein content in feed.

"Stocking density" refers to the number of fish in the pond, where the pond is a given size measured in hectares. (One hectare is equal to 10,000 square meters.) The more fish in an area, the greater the demand for oxygen.
The amount of "dissolved oxygen" in the pond can be increased with the use of aeration paddles, but it costs money to run them. You will want to maximize the number of fish in the pond without the need to add oxygen at costs higher than the profit you make from the fish. Dissolved oxygen is usually measured in milligrams (about 1/1000ths the mass of a paperclip) per liter. (In this simulation, you will see the units in milligrams per dekaliter, which is 10 liters and abbreviated "mg/daL".)
The other factor that will influence your profit is how fast your fish will grow. Fish, like all animals, need nutrients such as nitrogen obtained from protein in their diet. Higher "protein content" in the food you give fish can increase fish growth, but it is more costly than low protein content feed. You will be able to manipulate the amount of protein in your fish food. Fish need nitrogen to make proteins, DNA, and RNA. Excess protein in the feed will cause high ammonia levels in the water. Protein content is given as a percentage within the overall food supply.