Dissolved Oxygen Calculations

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Part 1: Multiple Choice & Quick Reasoning

1. Which of the following is NOT a major factor affecting dissolved oxygen (DO) in a small aquaponic tank?

Water temperature

Surface agitation from the filter return

Fish stocking density

Colour of the tank stand

2. At 20°C the saturation DO is ≈ 9.1 mg/L. If a cascading filter maintains 80% saturation, the estimated DO is about:

6.4 mg/L

7.3 mg/L

8.2 mg/L

3. According to water-quality categories, which DO range is labelled "Acceptable" for many species?

0–2 mg/L

3–5 mg/L

5–7 mg/L

9–11 mg/L

4. Check all management factors that generally INCREASE dissolved oxygen during the daytime:

Cooler water temperature

Increased surface agitation (larger waterfall)

Higher fish stocking density

More plants in the system (daytime photosynthesis)

Part 2: Calculations & Short Answers

5. Scenario A — Estimate DO and total oxygen

Tank: 30 L, water temp 22°C. Use DO_saturation ≈ 8.8 mg/L at 22°C and assume the cascade/filter returns water giving 80% saturation.

a) Calculate the estimated DO in mg/L. Show working.

b) Calculate the total dissolved O₂ in the tank in mg (DO mg/L × volume L).

6. Scenario B — Range estimate

Tank: 30 L, water temp 25°C. DO_saturation ≈ 8.3 mg/L at 25°C. A cascade can maintain 70–90% saturation.

Calculate the DO range (mg/L) for 70% and 90% saturation, then calculate the corresponding total O₂ in mg for the 30 L tank.

7. Effect of flow rate on DO (numeric)

At 20°C DO_saturation ≈ 9.1 mg/L. If a low flow gives 75% saturation and a higher flow raises this to 88%, calculate the change in DO (mg/L).

8. Atmospheric pressure and DO — short answer

9. Management decision (two parts)

a) A tank is measured at 5.5 mg/L DO at 24°C with moderate stocking. Is this DO level likely to be acceptable, stressful, or dangerous? Briefly justify your answer in one sentence.

b) Choose two actions that would most quickly increase DO in this tank:

Add an air stone / bubbler

Increase the height/flow of the filter return (more surface agitation)

Add more fish to the tank

Add extra plants (note: plants increase DO only during daylight photosynthesis)

Dissolved Oxygen Calculations

Dissolved Oxygen Calculations

Part 1: Multiple Choice & Quick Reasoning

Part 2: Calculations & Short Answers

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