The Critter Tank and the Nitrogen Cycle

At the National Marine Life Center, one of the interns’ responsibilities is taking care of the “critter tank” in the Discovery Center. The critter tank is a great opportunity for kids to come see some of the common coastal animal and plant species that live in Cape Cod. A couple weeks ago, Brittany and I walked the shores of Onset beach and gathered a number of interesting species to fill the tank with. This was my first beach collection experience, as previously I’ve lived far from the ocean, and it was a blast! You may have read about this experience in Brittany’s blog last week. We caught a large number of different things but had to limit what we actually put into the tank. Why? We had to give the water in the tank the opportunity to build up the nitrogen cycle.
For those familiar with biology, the nitrogen cycle might bring up thoughts of farm fields and rivers, but we’re talking about a different type of nitrogen cycle here. It takes some time for the water quality in a tank to be stable enough to sustain large numbers of different animals.

When you feed your fish in the tank, they produce waste and ammonia. This ammonia can also come from the decay of excess food in the tank or dead pieces of plant material. Ammonia is toxic to fish in high levels, but luckily we have a type of good bacteria called Nitrosomonas that exists in your fish’s water that will digest ammonia (NH3) and break it into nitrite (NO2).
Nitrite is slightly better than ammonia for your fish – and can be deadly in high levels, so we need another bacteria friend to take the Nitrite (NO2) and make it into Nitrate (NO3), this bacteria is called Nitrobacter.
Nitrate isn’t good for your fish either, but luckily it takes high levels of it to affect your fish poorly. Nitrate is used by the plants that live in your aquarium, and you can keep toxic levels from developing by doing routine partial water changes to your tank.
Well if my tank contains these good bacteria, why do I have to wait to stock my tank to capacity? Unfortunately, it takes these bacteria awhile to divide and reproduce to a high enough number to be useful. So, putting in a few fish gives the bacteria something to eat and energy to reproduce while not overwhelming your tank and killing your fish.

At the National Marine Life Center, we monitor these levels with weekly water quality tests. We test for nitrite, nitrate, and ammonia as well as pH and salinity (amount of salt in the water). Once we get the numbers from the test, we check to make sure that they are within safe parameters for the animals that live in the water. What happens if the chemical levels are too high or too low? We completely empty out the tank and refill it with new water, and retest before putting the animal(s) back into the tank.
We don’t just check water quality for the critter tank – we do it for all the turtle tanks, the pump house pulls salt water from the canal and the hospital tanks. The interns have been shadowing volunteers doing this task for a couple weeks, but we were able to complete all the tests on our own last week!

So, at the moment the critter tank contains a couple Asian Shore Crabs, and invasive species in Massachusetts. Invasive species are species that aren’t native to an area that they inhabit, and so may not have natural predators. They become a problem when they multiply to numbers that compete with native species for food and space. As cute as they are, they’re not a welcome visitor in Cape Cod waters.

The most valuable part of the critter tank to me is that kids can see animals interacting with one another in ways they can’t at the beach. These animals are relatively comfortable in the warm salt water we provide, and the kids can watch them move around, eat, and generally pester each other. The educational value of this is off the charts, not to mention fun! At the end of the summer these animals will be released back into the wild, to live out the rest of their lives in their native waters. Please come see our animals in the critter tank in our Discovery Center – it’s open every day from 10-5, has free admission and once July gets here, we will present daily educational programs for them! Stay tuned for more information!