If your koi are suddenly rubbing against the pond walls, bottom, or objects, this behavior, called flashing, is one of the most important early warning signs of a problem.

Flashing is not a disease itself.

It’s a symptom, and it usually means your koi are experiencing irritation or discomfort.

The key is acting quickly because early intervention can prevent serious illness or death.

What Is Flashing in Koi?

Flashing looks like:

• Sudden darting or jerking movements

• Rubbing sides against rocks, liner, or bottom

• Twisting or flicking motions

• Quick bursts followed by normal swimming

Occasional flashing can be normal.

Frequent or aggressive flashing is not.

Most Common Causes of Flashing

1. Parasites (Most Common Cause)

This is the #1 reason koi flash.

Common parasites include:

• Ich (white spot disease)

• Flukes (gill or skin)

• Costia

• Trichodina

• Chilodonella

Why parasites cause flashing:

They attach to skin or gills causing:

• Irritation

• Tissue damage

• Excess slime coat

Your koi rub against surfaces trying to remove them.

2. Poor Water Quality

Even if parasites aren’t present, bad water can irritate koi.

Key culprits:

• Ammonia spikes

• Nitrite poisoning

• pH swings 

• Low oxygen levels

3. Chemical Irritation

Anything added to your pond can trigger flashing in koi:

• Dechlorinator overdose

• Algaecides

• Herbicides

• Runoff from rain or lawn treatments

• Heavy metals (tap water issue)

4. Temperature Changes

Rapid temperature shifts stress koi and can cause:

• Increased parasite activity

• Immune suppression

• Behavioral stress (including flashing)

This is especially common in spring and fall.

5. Early Bacterial Infection

Sometimes flashing appears before visible symptoms like ulcers.

Look for:

• Red streaks in fins

• Slight skin irritation

• Clamped fins

6. New Fish Introduction

New koi often bring 

• Parasites

• Stress to the system

If flashing starts after adding fish, assume contamination until proven otherwise.

When  Koi Flashing Is an Emergency

Act immediately if you see:

• Constant flashing (not occasional)

• Fish isolating themselves

• Gasping at the surface

• Clamped fins

• Lethargy

• Visible lesions or white spots

These indicate a progressing health issue.

How to Diagnose the Cause

Test Your Water FIRST

Always rule out water issues before treating fish.

Test for:

• Ammonia (should be 0)

• Nitrite (should be 0)

• Nitrate (low)

• pH (stable)

• Temperature

If anything is odd, fix water before using medications.

Observe the Fish Closely

Look for:

• White spots: likely ich

• Excess slime: parasites

• Rapid gill movement: gill parasites or low oxygen

• Red streaks: bacterial issues

Scrape and Scope (Advanced)

The only way to confirm parasites is:

Microscopic examination

This identifies:

• Flukes

• Costia

• Trichodina

• Chilodonella

Without this, treatment is often guesswork.

How to Treat Flashing Koi

Treatment depends on the cause.

If It’s Water Quality

1. Perform a partial water change (10-25%)

2. Add dechlorinator

3. Increase aeration

4. Stop feeding temporarily

5. Monitor daily

If It’s Parasites

Treatment varies by parasite, but common options include:

• Salt (low-level use for mild cases)

• Praziquantel (flukes)

• Formalin/malachite green (Ich, Costis, etc.)

• Potassium permanganate (advanced users)

Always:

• Diagnose if possible

• Follow dosage exactly

• Remove carbon from filters

If Cause Is Unknown

Start with the safest approach:

1. Test and correct water

2. Add aeration

3. Observe 24-48 hours

4. Avoid random medication use

Blind treatment can worsen the situation.

How to Prevent Flashing

Prevention is far easier than treatment.

Maintain stable water

• Regular testing

• Proper filtration

• Avoid overstocking

Quarantine new fish

• Minimum 2-4 weeks

• Observe for parasites/disease

Seasonal awareness

Spring= parasite awareness

Fall= weakened immune systems

Feed high quality food

• Supports immune system

• Reduce stress

Observe your fish daily

Early detection is everything.

Related Problems You Should Know

Flashing often connects to:

• Ich (white spots)

• Flukes (gill irritation)

• Ammonia poisoning

• Dropsy (advanced cases)

• Low oxygen events

Final Thoughts

Flashing is one of the earliest and most important warning signs your koi will give you. If you see it, don't ignore it, check your water, and investigate quickly. Catching the problem early can mean the difference between a simple fix and a serious outbreak.

Frequently Asked Questions 

Why is my koi fish rubbing on the sides and bottom?

This behavior is called flashing, it can be a sign of an illness or pond issue.

Is occasional koi flashing normal?

Yes, rare and brief flashing can happen. Frequent flashing is not normal.

Can koi flashing go away on its own?

Only if caused by minor irritation. Parasites and water issues will worsen without intervention.

Should I treat koi without diagnosis?

Not ideal. Start with water testing before using medication.

Do all flashing koi fish have parasites?

No, but parasites are the most common cause.