Dissolved Oxygen Experiments
Like all animals, fish breathe oxygen.  You probably know that the air you breathe is not pure oxygen.  In fact, it is mostly nitrogen gas (N2) with oxygen gas (O2) dissolved in it.  Similarly, oxygen is found dissolved in most water, especially where there is agitation on the surface.  Rivers typically have higher oxygen levels than lakes or ponds because the surface of the water is in constant motion.  On windy days, lakes and ponds will have higher oxygen levels than on still days, again because of the agitation at the surface.  Small ponds, such as the ones used in fish farming, tend to have low oxygen levels.  Your Aunt Pearl has installed "aerators" to stir up the surface of her ponds.  Running the aerators is expensive, but without them, many fish would die due to lack of oxygen.

Your goal in this set of experiments, holding stocking density and protein content values constant, is to try a range of values for dissolved oxygen, where the aerators turn on.  If the value you select is too low, the aerators will not run often enough, and fish will start to die.  If the value you choose is too high, you will spend more money than you can afford running the aerators.  Your profits will disappear into thin air!

Again, you will want to try a range of values.  Looking at the resulting profits, you should again sketch a graph in order to determine approximately where the optimum value might be.  You should then try a narrower range of values near that possible optimum, in order to refine your value.

Oxygen levels are generally measured in the units of milligrams per liter (mg/L) or parts per million (ppm), but in this simulation we use milligrams per dekaliter (mg/daL).  That is a mass of oxygen in each volume of water.  The range you will want to work with is between 1 mg/daL and 100 mg/daL.  If the oxygen level drops to zero, all the fish will die.  Under normal conditions, water won't hold more than about 100 mg/daL in solution, no matter how much you run your aerators.

Note that you might get some higher profits with higher oxygen levels.  However, when you're looking for an optimum value, you may find that the high profits are harder to achieve consistently when those aerators are running regularly.  If you do select an oxygen level above 50 mg/daL as your optimum, it will be a good idea (**problem solving strategy hint**) to come back and try some lower values once you've identified all three of your optimum values.