The River Tank

The pH of aquarium water is another important measure because fish are adapted to living within specific pH ranges; bacteria are sensitive to pH levels as well. African cichlids, for example, prosper in alkaline waters with a pH range of 7.4-8.0, while certain tetras thrive in acidic waters with a pH range of 6.0-6.2. If a fish is forced to live in a pH level outside its preferred range, its slime coat can suffer, making it susceptible to disease. Its fecundity drops and, ultimately, the gas exchange in the gill membranes will be so reduced that the fish may suffocate.

Nitrosomonas and Nitrobacter prefer an alkaline environment (pH 7-8), and pH levels much below 6 severely curtail their activity or kill them. Once the bacteria are gone, the toxic ammonia quickly builds up to levels that will kill all the fish.

In an aquarium, acids derive primarily from two sources. The first is when carbon dioxide (directly dissolved into aerated water or released as a respiration by-product) mixes with water to form carbonic acid.

H₂O + CO₂ <=> H₂CO₃ <=> H⁺ + HCO₃^-
water + carbon dioxide <=> carbonic acid <=> hydrogen ion + bicarbonate

The other is when ammonia undergoes nitrification by Nitrosomonas.

2 NH₃ + 3 O₂ -> 2 NO₂^- + 2 H⁺ + 2 H₂O
ammonia + oxygen -> nitrite + hydrogen ion + water

Should any part of the tank become anaerobic, the heterotrophic, bacteria producing, organic, (eg., lactic, acetic, and formic) acids will add to the acid load of the tank and lower its pH through fermentation.

An underappreciated source of periodic pH swings is photosynthesis. During illumination, plants take in carbon dioxide, raising pH as the amount of carbonic acid (one source of carbon dioxide) in the tank is reduced. At night, respiring plants release carbon dioxide; consequently, the amount of carbonic acid in the tank increases, lowering the pH.

Water hardness is an often overlooked though extremely important component of pH balance in an aquarium. Water hardness refers to the total concentration of calcium and magnesium ions in the water, primarily from calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). These ions, called buffers, are important because they slow the rate at which the pH changes.

Figure 6 The River Tank ecosystem provides a convenient working model of a stream. It enriches a broad number of science topics and serves as a source of problem-solving challenges. Shown here is a 20-gallon tank.

Figure 7 Marsilea, an aquatic, cloverleaf fern, grows well in the moist soil or shallow water of a River Tank.

The equation below shows that carbonic acid (H₂CO₃) dissociates into hydrogen (H⁺) and bicarbonate (HCO₃^-) ions. The bicarbonate ions can further dissociate into hydrogen (H⁺) and carbonate (CO₃^-) ions. When acid (H⁺) is introduced into well-buffered water, carbonate ions react with the hydrogen ions to produce bicarbonate. Thus, even though acid is added, no change in the overall pH occurs. Furthermore, bicarbonate ions act as an additional reservoir for hydrogen ions. The reactions outlined in the equation below are pH sensitive and shift to the right as pH increases.

H₂O + CO₂ <=> H₂CO₃ <=> H⁺ HCO₃^-
<=> 2H⁺ + CO₃
water + carbon dioxide <=> carbonic acid <=> hydrogen ion + bicarbonate <=> hydrogen + carbonate

If the aquarium water is not well buffered (5.6-11.2 dKH or 100-200 ppm calcium carbonate in a freshwater tank), any acid that is added serves to drive down the pH. Consequently, the daily pH swings caused by photosynthesis can combine with longer-term acid accumulations and cause the tank to suddenly crash because of catastrophically low pH levels.

Oxygen is required to keep fish healthy and active and to maintain an aerobic environment that will support a robust, aerobic bacterial colony. The River Tank's waterfalls and rapids aerate the water and keep it well oxygenated. A comparison of the oxygen levels between the River Tank and a container of standing water would demonstrate the role agitation and mixing play in maintaining oxygen levels. The agitation also helps dissipate any chlorine in the water which escapes at the surface as chlorine gas.

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