How to Calculate a Redfield Ratio

Purchase nitrate testing kits and phosphate testing kits at the aquarium store and follow the instructions on the box. You should take readings in several different places -- top and bottom -- and average the results. The results should both be in milligrams per liter.

Divide the nitrate reading by the phosphate reading to get the Redfield ratio. Aquarium owners are only interested in the last part of the ratio -- the ratio of the nitrogen to potassium, which should be 16 to 1. You do not have to get this ratio exact to have a healthy aquarium -- as long as the ratio is between 10 and 22 to 1. Ratios below 10 to 1 usually lead to the growth of blue-green algae, and ratios over 22 to 1 usually lead to the development of green algae.

Adjust the Redfield ratio by adding potassium nitrate if the Redfield is low or by adding potassium phosphate if the Redfield ratio is high. If both the nitrogen and potassium readings are high -- no matter what the ratio is -- you need to change the water.

Correct the problem that is causing the imbalance if the Redfield ratio is off every month. Common causes include an imbalance of plants and fish and overcrowding -- too many fish for the size of the aquarium. Putting in a scavenger like a snail or a placastomous will often help. If these remedies do not correct the Redfield ratio, perhaps it is something in the tank that is causing the imbalance. The tiny diver, treasure chest or castle may be releasing something bad into your aquarium. Take it out for a while and see if anything changes.