When a koi starts flashing, clamping its fins, producing excess mucus, breathing heavily, isolating itself, or acting unusual, parasites may be one possible cause.

However, many koi health problems can look similar. Poor water quality, low oxygen, chemical irritation, bacterial infections, and parasites can all cause similar symptoms.

That is why it is important to diagnose before treating.

A skin scrape and microscope exam, commonly called a scrape and scope, can help determine whether external parasites are actually present.

A proper skin scrape does not involve cutting the fish or removing scales. The goal is simply to collect a small amount of mucus from the skin and examine it under a microscope.

Check the Water First

Before catching the koi, test the pond water.

At minimum, check:

Ammonia
Nitrite
Nitrate
pH
KH or alkalinity
Water temperature

If possible, also check dissolved oxygen, especially if fish are gasping, breathing heavily, or gathering near waterfalls or returns.

Symptoms such as flashing, clamped fins, lethargy, excess mucus, and rapid breathing do not automatically mean parasites are present.

If poor water quality is the real problem, parasite medication will not fix it.

When Should You Scrape and Scope?

A scrape and scope may be useful when koi show symptoms such as:

Flashing or rubbing against surfaces
Repeated jumping
Clamped fins
Excess mucus
Gray or cloudy skin
Unexplained redness
Lethargy
Loss of appetite
Sitting on the bottom
Isolating from other fish
Rapid breathing
Gasping at the surface
Hanging near waterfalls or returns
Unexplained sores or ulcers
Several fish showing similar symptoms

These signs do not confirm parasites. They simply indicate that further investigation may be needed.

What You Will Need

Fish Handling Equipment

Koi net
Koi inspection bowl or tub
Pond water
Wet nitrile gloves

Sampling Equipment

Microscope slides
Coverslips
Pipette or dropper
Small container of pond water
Paper towel
Smooth plastic scraping tool or coverslip

Microscope

A basic compound microscope capable of approximately:

40X
100X
400X

is suitable for examining koi mucus samples.

With a typical 10X eyepiece:

4X objective = 40X total magnification
10X objective = 100X total magnification
40X objective = 400X total magnification

Using several magnification levels allows you to scan the entire sample at low power and then examine smaller details more closely.

Prepare Before Catching the Fish

Have everything ready before you catch the koi.

Turn on the microscope, prepare your slides, fill the inspection bowl, and make sure your sampling tools are nearby.

The goal is to keep handling time as short as possible.

Koi should be handled gently and returned to the water promptly after the sample is collected.

Depending on the size of the koi, it may be helpful to have a second person assist you. Larger koi can be strong and difficult to hold safely, so having one person gently control the fish while the other collects the sample can make the process safer and less stressful for both the koi and the handler.

Step 1: Choose the Right Fish

If several koi are showing symptoms, choose one that is actively displaying the problem.

For example, if only a few fish are flashing, scrape one of those fish instead of choosing a koi that appears completely healthy.

If possible, examine more than one symptomatic fish.

Parasites may not be evenly distributed between fish or across the body of a single fish.

A negative scrape does not automatically mean parasites are absent.

Step 2: Catch and Bowl the Koi

Use a koi net to slowly guide the fish.

Avoid chasing it around the pond.

Transfer the koi into an inspection bowl containing pond water.

If you need to hold the fish, use wet nitrile gloves and gentle pressure.

For larger or stronger koi, having another person assist can make handling much easier. One person can focus on keeping the fish calm and secure while the other takes the mucus sample.

Never allow the fish or its slime coat to dry out.

Step 3: Choose Where to Scrape

Good sampling areas include:

Behind the gill plate
Along the shoulder
Along the side of the body
Near the base of the pectoral fins
Along the flank
Near the tail
Areas with unusually heavy mucus

Collecting samples from more than one area can improve your chances of finding parasites.

Avoid scraping directly through open sores or ulcers. If needed, sample mucus from the surrounding skin.

Step 4: Collect the Mucus Sample

Hold the coverslip or smooth scraping tool at a shallow angle against the fish.

Gently move it:

From the head toward the tail.

Do not scrape backward against the scales.

You only need enough pressure to collect a small amount of mucus.

Think:

Collect mucus, not skin.

A proper sample usually looks like a thin streak of clear or cloudy mucus.

You should not see:

Blood
Torn skin
Missing scales

If you are removing scales, you are pressing too hard.

Step 5: Return the Koi to Water

Once the sample is collected, return the koi to the bowl or pond.

Do not keep the fish out of the water while examining the slide.

You can always collect another sample if needed.

Step 6: Prepare the Slide

Place the mucus onto a clean microscope slide.

Add a small drop of pond water if the sample needs more moisture.

Gently place a coverslip over the sample.

Do not press down hard.

The goal is to spread the mucus into a thin layer without crushing organisms.

Avoid using:

Distilled water
Deionized water
Reverse-osmosis water
Chlorinated tap water

Pond water is the simplest choice.

Too Much Water?

Touch the corner of a paper towel to the edge of the coverslip to wick away the excess.

Too Little Water?

Add a small drop of pond water to the edge of the coverslip.

Examine the Slide Right Away

Fresh samples are easier to interpret.

Movement is often an important part of identifying what you are seeing.

As the sample dries, organisms may slow down or die, making identification more difficult.

Prepare the sample and examine it immediately.

How to Use the Microscope

Start at 40X

Always begin at low magnification.

A 4X objective with a 10X eyepiece gives approximately:

40X total magnification

This gives you a wide field of view and makes it easier to scan the entire sample.

Do not begin at 400X.

At high magnification, you are looking at a very small part of the slide and can easily miss organisms elsewhere in the sample.

Scan the Entire Sample

Move across the slide in an organized pattern.

For example:

Start in one corner.
Move slowly across the slide.
Drop down slightly.
Move back in the opposite direction.
Continue until the entire sample has been checked.

Think of it like mowing a lawn.

Randomly moving around the slide makes it easier to miss something.

Adjust the Light

Many organisms found in mucus samples are nearly transparent.

Too much light can make them harder to see.

Reducing the microscope light or closing the iris diaphragm slightly can improve contrast.

Move Up to 100X

After scanning the slide at 40X, move to approximately:

100X total magnification

This gives you a closer view of anything suspicious you found during the initial scan.

If you find something interesting, center it in the field before increasing magnification.

Use 400X for Very Small Organisms

For very small organisms or details that are difficult to see at lower magnification, move to approximately:

400X total magnification

Carefully focus through different layers of the mucus rather than looking at only one focal plane.

This is especially important when examining very small or nearly transparent organisms.

Not Everything Moving Is a Parasite

A skin scrape contains more than parasites.

You may see:

Mucus
Skin cells
Algae
Organic debris
Bacteria
Air bubbles
Small particles

Some debris may appear to move because water is flowing underneath the coverslip.

Do not identify something as a parasite simply because it moves.

Look for:

Consistent shape
Characteristic movement
Recognizable internal structures
More than one example of the same organism

Experience makes identification much easier.

For help identifying what you see under the microscope, refer to our separate guide on common koi parasites and how to identify them.

What if You Find Nothing?

A clean scrape does not guarantee that the fish is parasite free.

There are several possible reasons.

You Sampled the Wrong Area

Try another location on the fish.

The Problem May Be Concentrated on the Gills

Some parasites are found more commonly on the gills and may not show up easily on a skin scrape.

There May Only Be a Few Parasites

A lightly infected fish may produce a negative sample.

Another Fish May Be Better to Examine

If several koi are showing symptoms, scrape another affected fish.

The Problem May Not Be Parasites

If repeated scrapes are negative, reconsider other causes such as:

Poor water quality
Low oxygen
Chemical irritation
Injury
Bacterial disease
Viral disease

Do not repeatedly medicate the pond simply because you expected to find parasites.

What About Gill Samples?

Gill examination can be very useful because some parasites concentrate on the gills.

However, collecting gill tissue is more invasive than taking a skin scrape.

Gill tissue is delicate and contains many blood vessels. Improper sampling can cause bleeding or injury.

Beginners should start with skin scrapes.

A gill examination may be worth considering when a fish shows:

Rapid breathing
Gasping
Coughing
Hanging near waterfalls
Severe respiratory distress
Abnormal-looking gills
Repeatedly negative skin scrapes despite strong signs of parasites

Gill clips or biopsies are best performed by someone experienced with koi health procedures or by an aquatic veterinarian.

Diagnose Before You Treat

Finding a parasite is only part of the process.

Before choosing a medication, also consider:

What was found under the microscope
How many organisms are present
Whether more than one type appears to be present
Pond volume
Water temperature
pH
KH
Ammonia
Nitrite
Oxygen levels
Overall condition of the fish
Filtration
Other medications already in the pond

Different parasites require different treatments.

A medication that works for one problem may do little or nothing for another.

That is why the best approach is Identify first. Treat second.

Clean Your Equipment

Parasites and other pathogens can move between ponds on wet equipment.

Clean and disinfect items such as:

Nets
Bowls
Sampling tools
Hoses
Measuring equipment

Be especially careful when working with quarantine tanks or newly purchased koi.

Quick Scrape and Scope Checklist

Test the water first.
Prepare your microscope and supplies.
Have a second person help if the koi is large or difficult to handle.
Choose a symptomatic koi.
Gently net and bowl the fish.
Use wet gloves and gentle handling.
Collect mucus from several areas.
Scrape from head toward tail.
Do not remove scales.
Return the koi to water immediately.
Place the mucus on a slide.
Add pond water if needed.
Apply the coverslip gently.
Examine the sample immediately.
Start at 40X.
Scan the entire slide.
Move to 100X for closer examination.
Use 400X for very small organisms or finer details.
Identify what you are seeing before choosing a treatment.

Final Thoughts

A microscope is one of the most useful tools a serious koi keeper can own.

Instead of asking:

"What should I treat the pond with?"

you can start by asking:

"What is actually causing the problem?"

That difference matters.

Test the water, observe the fish, scrape and scope when appropriate, and identify the problem before reaching for medication.

A microscope cannot answer every koi health question, but it can replace a lot of guesswork with something much more useful ...evidence.

For severe illness, unexplained deaths, major gill problems, or cases that cannot be confidently diagnosed with a routine skin scrape, consult an aquatic veterinarian or experienced koi health professional.