Fill an aquarium with seawater. Aquariums are isolated water bodies ideal for cultivating algae on your own. Locate a pond if an aquarium is unavailable. A pond within a greenhouse is ideal, as a greenhouse allows you more control to actively modify environmental conditions. A small pond -- 25-by-25 feet, for example -- is best for isolating the algae to grow.
Dip a pH test strip into the water. Levels must stay between 7 and 9 pH for proper algae cultivation.
Gather a 1 L pitcher of aquarium or pond water. This water will act as a test sample for salinity measurement.
Turn on your electrical conductivity tester -- a tester measuring water flow and salinity readings -- and dip the probe into the pitcher. Move the probe around in the water for 10 seconds. Pull the probe from the water and examine the reading. The water must present between 20 and 40 g of salt per liter of water for algae to grow. Salinity is the amount of salt in the water.
Place a digital water thermometer probe into the actual body of water you will use to cultivate the algae. Remove the probe when the thermometer beeps. Read the temperature. Algae cultivation requires water to maintain temperatures between 64 and 75 degrees F at all times.
Expose the water source to natural daylight between 16 and 24 hours a day. Expose the water source to darkness for a maximum of eight hours per day. Under these physical conditions, if algae are present in the water, the algae should begin to cultivate with one week.