How to Do a Koi Skin Scrape: Parasite Diagnosis

By KoiQuanta Editorial Team|Updated Mar 2026

A single skin scrape misses parasites if taken from the wrong body area -- protocol matters. This is the most common reason hobbyists do skin scrapes and conclude "no parasites found" on a fish that's actively symptomatic with flashing and excess mucus. A scrape from a clean area of the fish's back won't show the fluke concentration that may be present on the flanks or under the pectoral fins.

Understanding where to scrape, how to prepare the slide, and what you're looking for turns a skin scrape from a guessing tool into a reliable diagnostic.

TL;DR

For most common external parasites (trichodina, flukes, costia, ich), 100-200x provides adequate visibility.
Some smaller organisms benefit from 400x.
You don't need a high-end research microscope -- a basic student microscope (40x-400x) from a hobby supplier is sufficient for all common koi parasites.
This typically takes 1-2 minutes at the correct dose.
Using a plastic scraper at 45 degrees, make one firm stroke from head to tail on the lateral surface below the dorsal fin, behind the pectoral fin, and along the ventral surface.
Transfer the collected mucus to a drop of pond water on a glass slide, add a coverslip, and examine immediately at 40-400x.
Trichodina and flukes are well-visible at 100-200x.

What You Need

A microscope: The minimum useful magnification for koi parasite diagnosis is 100x. For most common external parasites (trichodina, flukes, costia, ich), 100-200x provides adequate visibility. Some smaller organisms benefit from 400x. You don't need a high-end research microscope -- a basic student microscope (40x-400x) from a hobby supplier is sufficient for all common koi parasites.

Glass slides and coverslips: Standard microscope slides (25x75mm) and coverslips (18x18mm or 22x22mm). Have a box of each.

A plastic scraper or dull blade: A blunt plastic scraper, a glass slide edge, or the back of a scalpel blade. You're scraping mucus, not cutting -- the instrument needs to be firm but shouldn't cut the fish.

A few drops of pond water: Place on the slide to dilute the scraping and keep organisms alive and motile. Motile organisms are easier to identify than fixed ones.

An anesthetic (optional but strongly recommended): Clove oil at sedation concentration (30-50 mg/L, approximately 2-3 drops in 10 liters) or a commercial fish anesthetic (MS-222 at appropriate dose). Sedated fish are dramatically easier to scrape correctly without causing injury.

KoiQuanta's diagnostic log records scrape results and links to the treatment protocol selected. Documenting which sites were scraped, what organisms were found, and their relative abundance creates a diagnostic record that helps you assess treatment efficacy over time.

Selecting the Sites to Scrape

The parasite load on a fish is not uniformly distributed. Where you scrape determines what you find.

Standard scrape locations:

Lateral surface below the dorsal fin: The flank. A good general site that gives you a representative sample of whatever's on the body.
Behind the pectoral fin: Parasites often concentrate in the protected space just behind the pectoral fin base. This is the most productive site for detecting low-level infestations.
Ventral surface (belly): Near the vent and along the midline. Useful for detecting costia and trichodina that prefer ventral surfaces.
Fin tissue: Scrape the edge of the dorsal or caudal fin where parasites often concentrate.

For gill parasite diagnosis, skin scraping is not sufficient -- gill biopsies are required. The koi gill scrape guide covers gill sampling for gill flukes and other gill parasites.

Performing the Scrape

If sedating: Add clove oil solution to a small bucket of pond water. Hold the fish gently with wet hands and allow it to lose balance (stage of sedation). This typically takes 1-2 minutes at the correct dose. Work quickly once sedated.

Positioning: Hold the fish gently but firmly with your non-dominant hand, or lay it on a wet surface. Keeping the fish wet is critical -- dried skin gives poor samples and damages mucus.

The scrape technique: Hold your scraper at about 45 degrees to the body surface. Apply light to moderate pressure and draw firmly from head to tail across the site. One firm stroke is better than multiple light passes. You're collecting the surface mucus layer.

Slide preparation: Place a drop of pond water on a clean slide. Transfer your scraping (the small amount of mucus and material on your scraper) to the water drop. Place a coverslip over the sample with a slight rolling motion to avoid air bubbles. Apply gentle pressure to flatten the coverslip and spread the material.

Immediately examine: Start with 40-100x to survey the field, then increase magnification for organisms you want to identify more closely. Work through the slide systematically rather than randomly -- examine from one end to the other.

What to Look For Under the Microscope

Gyrodactylus (skin flukes): Oval, elongated worms about 0.5-1mm long (very small but just visible to the naked eye at high density). Under the microscope, look for anchors (hooks) at the anterior end and a live birth structure (grasping hooks and embryo visible inside the adult). Gyrodactylus are viviparous -- they give live birth.

Dactylogyrus (gill flukes): Similar shape to Gyrodactylus but has four eyes (Gyrodactylus has no eyes) and a characteristic head organ. Usually found in gill scrapes rather than skin scrapes, but occasionally found on skin.

Trichodina: Saucer-shaped, circular, with a ring of teeth (denticles) visible at 100-200x. Look for the characteristic spinning motion as the organism moves. The denticle ring distinguishes trichodina from other rounded organisms on the slide.

Costia (Ichthyobodo): Very small (7-15 microns), pear-shaped flagellated protozoa. Look for rapid tumbling motion. You need 400x to see these clearly. They're often attached to the mucus by a sucker and detach rapidly -- examine immediately after preparation.

Ich (Ichthyophthirius): Large, distinctive organism visible at 40-100x. Rounded, with cilia around the perimeter and a horseshoe-shaped nucleus. Much larger than trichodina -- if you see very large, clearly cilia-covered spheres, this is ich.

Chilodonella: Oval, flattened, about 50-70 microns long. Characteristic band of cilia along one side and spiral nuclear arrangement. Moves with a slow gliding motion.

Anchor worm (Lernaea): The embedded portion that's below skin surface can sometimes be seen in a scrape as tubular structures with an attachment structure. Usually identified visually on the fish surface rather than by scrape.

Quantifying What You Find

Finding one or two organisms on a slide is not necessarily clinically significant -- some level of parasite presence is normal in most pond fish. You're looking for:

High numbers: Dozens to hundreds of organisms per field of view indicate heavy infestation
Multiple organisms per field: Even 5-10 trichodina per field at 100x suggests a significant population
Finding the same organism on multiple scrape sites: Consistent finding across sites indicates widespread infestation

A negative scrape (nothing found) doesn't definitively rule out parasites -- as noted above, scraping location matters. If symptoms are strongly suggestive, scrape multiple sites including gill tissue before concluding negative.

For what microscope specifications to look for, the koi microscope guide covers minimum specifications for koi diagnostic work. For the gill biopsy procedure that complements skin scraping, see the koi gill scrape guide.

Frequently Asked Questions

How do I do a koi skin scrape?

Sedate the fish briefly with clove oil (30-50 mg/L) for safe handling. Using a plastic scraper at 45 degrees, make one firm stroke from head to tail on the lateral surface below the dorsal fin, behind the pectoral fin, and along the ventral surface. Transfer the collected mucus to a drop of pond water on a glass slide, add a coverslip, and examine immediately at 40-400x. Document what organisms you find, their approximate density per field, and which sites were scraped. Link your findings to the appropriate treatment protocol in KoiQuanta.

What microscope do I need for koi diagnostics?

A basic student microscope with 40x, 100x, 200x, and 400x objective capability is sufficient for all common koi external parasites. Trichodina and flukes are well-visible at 100-200x. Costia requires 400x for clear identification. You don't need a phase-contrast or fluorescence microscope -- standard brightfield illumination at these magnifications is adequate. Prices for adequate microscopes start around $100-150 for entry-level student models and $300-500 for better quality instruments with good optics.

What do I look for in a koi scrape sample?

At 100x, survey the full slide systematically: oval worms with hooks (flukes), circular saucer-shaped organisms with denticle rings (trichodina), and large cilia-covered spheres (ich). At 400x, look for small pear-shaped rapidly moving organisms with flagella (costia). Note the density of any organisms found -- a few incidental organisms are different from dozens per field. Compare across your multiple scrape sites. Consistently finding the same organism at high density across multiple sites warrants treatment for that specific parasite.