Often the most important changes in an aquatic environment are chemical changes. A River Tank is a good model system for students to investigate changes in water chemistry. First, using the tank eliminates the need for parent/guardian permission and problems associated with a field trip to a stream. Second, the tank makes it easy to manipulate factors that alter water chemistry. All you need are a River Tank ecosystem, gravel, plants, animals, and test kits or meters (probes) to detect chemical changes in the water. Among the parameters you might want students to test are pH; water hardness; and levels of ammonia, nitrite, and nitrates.
- pH—Most healthy freshwater ecosystems have a pH range of 6.8 to 7.8. Common factors that can influence water pH include levels of dissolved carbon dioxide, ammonia, and calcium carbonate. Fish, other aquatic animals, and even decomposing organic matter all release carbon dioxide into the water. As the carbon dioxide level rises, pH drops (becomes more acidic). Aquatic animals also commonly release waste products, in the form of ammonia, into the water. As the ammonia level rises, so does pH. However, the presence of calcium carbonate (CaCO3) tends to buffer water against pH changes. Limestone gravel is a common source of calcium carbonate for aquarium water.
- Nitrogen—Ammonia (NH3) is toxic to fish and most other aquatic organisms. Removal of ammonia takes place naturally through the nitrogen cycle or nitrification. In this cyclic process, Nitrosomonas bacteria break down harmful ammonia from animal waste, plant debris, and decomposing food into nitrite (NH2), which is less harmful. Other bacteria of the genus Nitrobactor then convert the nitrite to nitrates. Plants use nitrates as a nitrogen source to synthesize proteins. Nitrates are also removed by regular partial water changes. The aquarist’s adage is, “The solution to pollution is dilution.”
- Water hardness—Water is “hard” or “soft” depending on the concentration of calcium carbonate (CaCO3) in it. The level of dissolved calcium carbonate is measured in parts per million (ppm). Most animals in aquatic environments absorb dissolved calcium to build bones or exoskeletons, which lowers water hardness. As noted above, calcium carbonate also helps to maintain the pH of an aquatic environment. Soft water has reduced buffering capacity and can lead to a rapid drop in pH. A level of 50–200 ppm of calcium carbonate, which is slightly to moderately hard, is best for most aquatic environments.
Set up your River Tank with substrate (gravel, either quartz only or with some limestone mixed in) and dechlorinated water. Turn on the pump and let it run for 1 or 2 min. Add more water if needed, but do not add plants or animals at this time. After the River Tank runs overnight, direct students to make and record their initial readings to serve as the baseline. After that, add the plants. Students can take readings twice a week if using water test kits, or daily or as often as desired if using meters.
Two weeks after you plant the River Tank, add animals and have students continue to take readings. Students should note possible correlations between adding plants and animals and changes in water chemistry. They also should note the addition of food, replacement of water lost by evaporation, and other changes in the River Tank community, looking for related changes in water quality. Using books and other resources, students can offer possible explanations for their observations.
- Test for other parameters such as dissolved oxygen, alkalinity, and total dissolved solids.
- When the readings for your River Tank stabilize, add 3 new fish. Does this alter the readings? Does the River Tank restabilize as before?
- Compare a River Tank to a standard aquarium with the same kind and number of plants and animals. If the standard aquarium does not use a pump and filter, students can compare a moving-water habitat to a still water habitat. Comparing the level of dissolved oxygen in the 2 systems can be especially interesting.
- Compare a River Tank with plants only to a River Tank with animals only.