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Why Are My Koi Gasping at the Surface?

Why Koi Gasp at the Surface

When koi are gasping, hanging near the surface, crowding around waterfalls, or breathing rapidly, it almost always means low dissolved oxygen.
This is one of the most common and dangerous pond emergencies, especially in summer or in ponds with heavy fish loads.

Koi go to the surface because oxygen levels are highest there.
This behavior is a warning sign — but the problem can often be fixed quickly with the right steps.

The Most Common Causes of Low Oxygen

✔ Hot weather

Warm water holds significantly less oxygen.
Once the pond hits 80°F+, oxygen levels can drop fast.

✔ Overfeeding

Excess food → extra fish waste → bacteria use oxygen to break it down.

✔ Poor aeration

A waterfall alone does not provide enough oxygen for most koi ponds, especially deeper ones.

✔ Algae blooms

Algae produce oxygen during the day…
…but consume oxygen heavily at night, sometimes enough to cause morning gasping.

✔ Killing algae too quickly in warm weather

When algae dies suddenly — from UV clarifiers, algaecide use, or extreme sunlight shifts — the decaying algae consumes a large amount of oxygen in a short period of time. The loss of oxygen producing algae all at once can also contribute to low oxygen levels. 
This often triggers gasping events within hours.

✔ Dirty filters or sludge buildup

Decomposing muck uses oxygen and lowers overall water quality.

✔ Overstocking

Too many koi increases oxygen demand beyond what the pond can produce.

✔ Medication or chemical treatments

Some treatments temporarily reduce oxygen levels.

✔ Pump or circulation issues

If your pump isn’t turning the pond volume over at least once per hour, oxygen can drop quickly.

Immediate Steps to Help Your Koi (Rapid Response)

These actions can raise oxygen levels within minutes.

✔ Add or increase aeration

Turn on or boost:

  • Air pumps

  • Air stones

  • Diffusers

  • Venturi aerators

If nothing else is available, place a small pump near the surface to splash and agitate the water.

✔ Increase surface agitation

If using a variable speed pond pump, turn up your waterfall to increase agitation. 

✔ Do a 10–20% water change

Use dechlorinated water.
Fresh, cool water helps raise oxygen levels quickly.

✔ Stop feeding for 24–48 hours

Digestion requires oxygen — pausing feeding reduces strain immediately.

✔ Keep all pumps and filters running

Good circulation is essential during a low-oxygen event.

Fish often improve within a few hours once oxygen levels rise.

Fix the Root Cause (Long-Term Prevention)

Improve Aeration

Every koi pond should have a dedicated aeration system. A waterfall alone is rarely sufficient.

Ideal setup:

  • A properly sized air pump

  • Multiple air stones or diffusers

  • Strong surface agitation

  • Deep aeration in warm weather / mid-depth aeration in winter

Better aeration = better oxygen levels, clearer water, and healthier koi.

Reduce Sludge and Waste

Decomposing organic matter consumes oxygen.

Use:

  • Mock Sludge Digester to break down muck naturally

  • A pond vacuum for heavy buildup

  • Regular netting of debris

Less sludge = more oxygen available for koi.

Add Beneficial Bacteria

A strong biological filter prevents ammonia and nitrite spikes and reduces organic waste.

Use the correct bacteria for your water temperature:

✔ 32–50°F: Mock Cold Water Formula

✔ Above 50°F: Mock 4-in-1 Formula

Weekly dosing helps maintain oxygen balance and reduces waste that bacteria must break down.

Control Algae Responsibly

Managing algae is important, but doing it too aggressively can cause oxygen crashes.

Safe algae management:

  • Add aquatic plants for shading

  • Improve filtration and circulation

  • Use a UV clarifier for green water

  • Remove debris and sludge

  • Avoid overfeeding

Sudden, massive algae die-offs should be avoided in hot weather.

Improve Water Circulation

Your pump should circulate the entire pond volume once per hour.

Signs of poor circulation:

  • Koi hanging only near the waterfall

  • Temperature differences in different sections

  • Dead spots or unmoving water

  • Persistent cloudy water

Good circulation ensures even oxygen distribution throughout the pond.

Adjust Feeding

Overfeeding is a major source of oxygen depletion.

Guidelines:

  • Feed only what koi eat in 2–5 minutes

  • Never feed below 50°F

  • Remove uneaten food

Better feeding habits = better oxygen stability.

Avoid Overstocking

A general guideline is:

1 inch of fish per 10 gallons of water.

Too many koi → too much ammonia and oxygen demand → frequent gasping events.

How to Recognize Low Oxygen Early

Watch for:

  • Koi staying near the surface

  • Rapid gill movement

  • Gathering near waterfalls or aerators

  • Gasping early in the morning

  • Refusing food

  • Lethargy or slow swimming

  • Red or irritated gills

  • Flashing or darting behavior

Early detection prevents emergencies.

When to Seek Professional Help

Call for expert assistance if:

  • Koi continue gasping after adding aeration

  • Fish lay on their sides or seem unresponsive

  • Ammonia or nitrite levels are present

  • Water is extremely cloudy or green

  • Oxygen crashes keep occurring

  • Equipment may be undersized or failing

Mock Pools & Ponds can test the water, evaluate your system, and recommend upgrades for aeration and filtration.

Healthy Oxygen Levels Keep Koi Safe

Once aeration, waste control, filtration, circulation, and feeding are balanced, koi breathe easily and the entire pond becomes healthier and more stable.

For aerators, pumps, beneficial bacteria, sludge digester, UV clarifiers, and personalized pond support, visit Mock Pools & Ponds, proudly serving koi owners throughout Canton, Akron, Medina, and Northeast Ohio.

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