Dissolved Oxygen in Fish Ponds: What Levels Are Too High or Too Low?

1. The dangers of too much dissolved oxygen

• A slight rise in dissolved oxygen is already harmful to egg hatching and fry, making them prone to gas bubble disease; even small fingerlings (summer fry) are affected.
• When dissolved oxygen is high, fish become excited and swim in circles continuously — small fry can literally exhaust themselves to death.

2. The dangers of too little dissolved oxygen

• When oxygen drops below a certain level, the metabolism and growth of aquatic animals start to suffer — without immediate death. This level is called the critical dissolved oxygen concentration.
• If oxygen keeps falling below the animal's minimum physiological need, the stock suffocates and dies. This level is called the lethal dissolved oxygen concentration.
• Both critical and lethal levels vary by species and size, and are affected by water temperature, salinity and other environmental factors. As water temperature rises, lethal oxygen levels change accordingly.

3. What is the normal range?

• Dissolved oxygen in culture water changes throughout a 24-hour cycle, and is lowest at night.
• General guideline: dissolved oxygen must stay above 4 mg/L for 8 hours (overnight), not below 5 mg/L for 14 hours, and never below 2 mg/L at any time.

4. Where dissolved oxygen comes from

• Natural absorption. On windy days, oxygen from the air dissolves into the water; the wind speed directly affects how fast this happens.
• Mechanical aeration. A Roots blower feeds air through diffuser discs laid on the pond bottom, aerating the whole water column from bottom to surface, enlarging the air-water contact area and promoting oxygen dissolution.
• Phytoplankton. Oxygen released by algae through photosynthesis is the main source of pond oxygen, contributing 80–90% of a pond's total dissolved oxygen.