Aquarium Algae and Fertilizer Runoff
Students maintain a 40-L classroom aquarium with six fish, twelve snails, and several plants. Hoping to improve plant growth, they add 5 mL of liquid plant fertilizer daily for two weeks. Soon a green film covers the glass, and some fish are gulping at the surface before school. A test strip shows nitrate at 40 mg/L and dissolved oxygen at 4 mg/L at 8:00 a.m.; by 2:00 p.m. oxygen is 7 mg/L. A comparable tank without fertilizer has nitrate at 10 mg/L and oxygen between 7 and 8 mg/L all day. The class concludes that fertilizer is a poison that directly harms fish and removes it immediately. The teacher asks the students to evaluate that conclusion and to explain why the oxygen reading changes during the day.
1. Construct a causal explanation for the low oxygen reading at 8:00 a.m. that uses the fertilizer, algae, and daily light cycle.
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The fertilizer adds nitrate, a plant nutrient. The tank already has algae, and the extra nutrient allows algae to grow rapidly and cover surfaces. During daylight, algae and plants photosynthesize and release oxygen, which is why the 2:00 p.m. reading rises to 7 mg/L. At night, photosynthesis stops, but algae, plants, fish, snails, and decomposing organisms continue to respire and consume oxygen. Decaying algae also feed bacteria that use oxygen. Oxygen therefore reaches its lowest point near dawn, after many hours without photosynthesis. The 8:00 a.m. reading of 4 mg/L fits this pattern, and the fish gulping at the surface is a behavioral sign of low dissolved oxygen. The fertilizer is best seen as a nutrient that triggers an algal bloom and an indirect oxygen problem, not necessarily as a direct poison.
2. Evaluate the conclusion that fertilizer directly poisons fish. What evidence supports or weakens it, and what additional evidence would help?
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The evidence supports a link between fertilizer addition and fish stress, but it does not prove direct poisoning. Nitrate is 40 mg/L in the fertilized tank versus 10 mg/L in the control, and oxygen is lower in the morning. However, no fish deaths, toxin measurements, or direct exposure tests are reported. The daily oxygen pattern is better explained by algal growth and nighttime respiration than by a simple toxic effect. Other possible causes include ammonia or nitrite buildup, overfeeding, or decaying plants. To test the claim, the class could compare tanks with and without fertilizer while keeping fish numbers, feeding, light, and temperature the same; measure nitrate, ammonia, nitrite, and oxygen at dawn and afternoon; and observe fish behavior. If oxygen remains adequate but fish still suffer, direct toxicity becomes more plausible. If removing algae and restoring oxygen solves the problem, the indirect explanation is stronger.
3. Propose a classroom follow-up that would test whether adding fertilizer, rather than another factor, causes the oxygen pattern.
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Set up two or more identical tanks with the same water volume, light schedule, temperature, plant mass, and number of fish or, more safely, no fish initially. Add the same volume of fertilizer to one tank and an equal volume of plain water to the control. Measure nitrate and dissolved oxygen at dawn and in the afternoon every day for two weeks. Also record algal cover and remove dead plant material consistently from both tanks. Replicate each treatment if possible and alternate tank positions to control for light differences. The prediction is that the fertilized tanks will show higher nitrate, more algal growth, and a larger dawn-to-afternoon oxygen swing than the controls. Keeping everything else equal is what allows the class to attribute any change to the fertilizer rather than to feeding, light, or temperature.