navy rule
navy rule

Mesocosm Home



Build It

Run It



More Info


Return to Munin Home

Copyright ©2000-06
P.D. Schreuders

Run It

navy rule


One of the most essential aspects of supporting a healthy ecosystem for the crayfish is to maintain optimum water quality. High levels of ammonia, nitrite, and nitrate are toxic for aquatic organisms. Also, extremely low levels of dissolved oxygen and high levels of carbon dioxide in the water will cause suffocation. The pH and alkalinity are also key factors to monitor. A few simple tests can be performed to ensure that the quality of water in the tank is adequate to promote crayfish survival. The procedures for all of the tests recommended on this site are listed here. The tests are designed to give you the ability to understand the cycles in your tank and how they effect the ecosystem. Through the data you collect, you will make changes in your tank, helping create a better ecosystem. Feel free to do any other tests besides these, these are just some important and interesting ones.

back to the top

navy rule

Daily Tests

Dissolved Oxygen

This test measures the amount of oxygen that is dissolved in the tank’s water. Crayfish require a dissolved oxygen level of at least 3 ppm or they will die or be injured. The ideal environment for crayfish is saturated with oxygen , so a good dissolved oxygen level is around 9 ppm. A low DO level could be caused by the lack of oxygen producing plants or heavy pollution that removes the oxygen from the water.

The test is performed with the use of a dissolved oxygen meter, which can be purchased from equipment dealers such as Omega and Eutech Scientific, Inc.

  • Take 50 to 100 mL of sample water
  • Calibrate the meter and insert it into the sample, swirling the water as it is measured.
  • Record the results.

Carbon Dioxide

  • Titrate tank water with a base (.00227 M Sodium Hydroxide) to find out how much carbon dioxide you have.
  • Take 100 mL of tank water and place it into a 125 mL flask.
  • Add 3-6 drops of phenolpthalien as an indicator
  • Stir in via titration sodium hydroxide until the sample has a pinkish tint that does not dissipate when stirred.
  • The amount of mL of sodium hydroxide titrated is equal to the mg/L of Carbon Dioxide.
  • Record the results

A high carbon dioxide level means the dissolved oxygen level is low, so typically a lower carbon dioxide measurement is preferred. The carbon dioxide level can go as high as 50 mg/mL and still have surviving crayfish, although the higher the level of carbon dioxide the less oxygen there is for the crayfish and this can hurt the crayfish.


This test can be performed by using water quality testing strips sold by many major scientific equipment catalogs such as Hach or a spectrophotometer. Make sure to follow the instructions carefully and record your results.

Ammonia is the most toxic form of aquatic nitrogen, it can poison the crayfish at relatively low concentrations . At 1 mg/L it begins to cause damage to the system and optimum levels are under 0.5 mg/L.


All of the gasses in the tank (Oxygen, Nitrogen, Carbon Dioxide etc;) effect the acidity of the water, the pH. For instance a high carbon dioxide measurement would indicate that there is a high pH. A pH higher or lower then the normal 6.5-9 can injure the crayfish and the plants in the tank.

  • The most accurate way to measure pH is a pH meter, like the one offered by Jenco. Other pH devices such as chemical treatment kits can be used although they less accurately determine the pH of the sample water. The following instructions pertain to the pH meter; consult the kits directions for other pH measurement devices for the kits protocol.
  • Place 100 mL of the tanks water into an Erlenmeyer flask.
  • Use pH 4 and 7 buffer solutions to calibrate the meter for any measurement taken - this step varies between meters, so consult the directions.
  • Rinse the probe with deionized water between each measurement, even during the calibration.
  • Place the clean probe into the sample water and measure the sample. The meter will indicate when a steady measurement has been obtained, record this stable value


Measure using an aquarium thermometer that can remain in the water and use consistent units. The ideal temperature for crayfish growth is around 22 C. Crayfish thrive in warmer temperatures, but their metabolism becomes more active then, increasing their waste production and decreasing the oxygen-carrying capacity of the water.

General observations about the behavior and appearance of the organisms should be made daily as well. For example, if the plants are turning brown this might indicate a oxygen/ carbon dioxide imbalance. Crayfish migrating to the top of the water may indicate low levels of oxygen in the tanks.

back to the top

navy rule

Tests performed 2-3 times a week

Nitrite and Nitrate

Like ammonia, these tests can be performed by a spectrophotometer or by water quality testing strips (sold by or any major scientific equipment catalog).

Nitrogen is found in the two forms of nitrite and nitrate (and ammonia). Nitrogen is broken up by bacteria in the nitrogen cycle to form these chemicals. They are not as toxic as ammonia and therefore only need to be monitored every three days.

Nitrite is less toxic then ammonia is and it should not exceed 4 mg/l.

Nitrate is much less toxic then ammonia or nitrite but it can build up in a system if the water is not changed periodically. Nitrate levels from 60 mg/L are considered fair for aquatic systems and the less nitrates the more excellent the water (Morgan, 1989).

Water Level

Maintaining a standard water level in your tank during the project is important. If the water level is rapidly decreasing, a major problem could be evident in your tank. Check for any leaks in the tank and make sure it does not go so low that the crayfish have little or no water above their heads. A greater amount of water is a good buffer against wastes overwhelming the system as there is more water for the substance to be spread out in. However too much water does not leave any room for trapping oxygen that can be incorperated into the water and if the water level is too high a crayfish might escape.


The change in alkalinity is more gradual and therefore only needs to be checked 2-3 days a week. Alkalinity is the systems ability to resist changes in pH which would disrupt the system, so a high alkalinity concentration is desired.

  • Place 100mL of sample water into a 125 mL flask.
  • Add 3-6 drops of bromocresol green - methyl red indicator to the flask
  • Titrate .02 M sulfuric acid until the sample turns from bluish green to a slightly pink tint

Record the milliliters of sulfuric acid required - each milliliter of sulfuric acid is equal to 10 mg/L CaCO3 in the sample water


Water that has become thick with sediment is turbid. In the tank this is caused by the excretion of waste and the growth of microbes in the water. It increases slowly, but can cause blockage of light in the tank, causing plants to die, taking away a food and an oxygen source for the crayfish. Turbidity also indicates other problems in the water such as high concentrations of organic matter that consume oxygen..

A spectrophotometer can be used to perform this test.

back to the top

navy rule

Tests performed every 1-2 weeks

Crayfish Biomass

Testing the mass of the crayfish gives you many insights into the amount of food and oxygen they need/have. Other factors like molting, fighting(loss of limbs), growth and reproduction also contribute to the biomass of the crayfish.

  • Place a container with water in it on a electronic scale (preferred) and zero the scale. This will make sure only the mass of the crayfish is measured and not the scale.
  • Place the crayfish in the container.
  • Place the container with the crayfish on a scale and record the results.

Plant Biomass

Lots of plants are needed to sustain the eating and breathing of the crayfish. If the biomass is decreasing, the crayfish are eating the plants faster then they can grow. To correct this problem at the sealing period, add more plants.

  • Zero the scale with a container on it
  • Dry as much of the water off of the plants that you can.
  • Place the plants in the container, measure and record results

back to the top

navy rule

Emergency Procedures

As with any design, certain unexpected problems may occur that warrant necessary emergency action. If one or more of the following events occur, any member of the group has the right to break the seal on the tank and rectify the situation:

  • DO level falls below 3 mg/L or two or more crayfish are at the water surface in search of air
  • The food source is completely diminished.
  • Ammonia levels exceed 1 mg/L and nitrite levels exceed mg/L
  • 2 or more crayfish are dead
  • pH level drops to less than 5.5 or greater than 10.0
  • H2O level drops to less than 2cm above crayfish

If any other unexpected situation presents itself, the group will discuss the options available to address the problem. Run tests on the tank to try and pinpoint the problem.

back to the top

navy rule

Temporary Fixes

While fixing the tank be sure to place your crayfish in a safe place and feed them food pellets until they are ready to be returned to the system.

  • To revive the dissolved oxygen count, drain the tank and replace with new dechlorinated water. This will replace the oxygen deprived water with new and unused oxygen rich water. To correct this problem permanently, you could add more plants or a stirring device that will mix air into the system. Also, you should think about not completely filling up the tank, leaving trapped air above the surface of the water, and this oxygen will infuse itself into the system. Also, having too many crayfish can cause problems with the level of oxygen (and most of the other levels in the tank) as they use it up too quickly.
  • More plants will serve as a consistent food source. Make sure you have enough plants for all of the crayfish.
  • Draining the tank and replacing the water will lower the nitrogen count. Also, remove any pieces of decaying matter (dead crayfish, dead plants) as they give off nitrogen.
  • If two or more crayfish are dead, its likely that a few other emergencies are going on in the tank. Do all the water quality tests and fix as needed.
  • Fix the pH by draining the tank and replacing the water with dechlorinated water. Remove any dead decaying matter because it can cause a drop in pH. Add c rushed coral to the gravel, as this will increase the alkalinity of the water to prevent future changes in pH (it also provides a good source of calcium for the crayfish).
  • If the tank begins to run out of water, simply fill it up again. However, you should check for leaks in the tank, and in the future when you test the water make sure not to take too much water out.

back to the top

navy rule