Guide
Scaleless fish: why they need you to think differently
Loaches, catfish, knifefish and elephantnose fish absorb what you put in the water faster than most community fish. Here is how to medicate, salt, decorate and handle them without losing them.
- "Scaleless" is a hobby label: it covers truly naked fish (many catfish), fish with tiny embedded scales (most loaches, knifefish, elephantnose, spiny eels) and armoured catfish with bony plates (corydoras, plecos). They are grouped because they react similarly to chemicals in the water.
- Medication: look for a scaleless warning on the label and follow that product's guidance. Don't invent a half dose for a product that doesn't mention one.
- Salt: use it cautiously at low levels, or leave it out. The evidence is mostly keeper experience.
- Substrate: smooth sand or rounded gravel. Sharp grit wears down barbels and skin.
- Treat smart: hospital tank where possible, one treatment at a time, carbon out during the course.
- They are your canaries: scaleless fish often show stress first. Take it seriously and test the water.
What "scaleless" actually means in the hobby
The word is used loosely. Medication labels and forum advice use it to mean "fish whose skin lets chemicals in more easily than a typical tetra or barb", and that covers three quite different groups.
Truly scaleless: the naked catfish
Many catfish have no scales at all, just thick, mucus-covered skin. Familiar examples include the pictus catfish (Pimelodus pictus), Synodontis species (a bony head shield, but bare skin on the body), glass catfish (Kryptopterus vitreolus) and banjo catfish. Their skin is also packed with taste receptors, so they are in close contact with everything dissolved in the water.
Tiny or embedded scales: loaches, knifefish, mormyrids and spiny eels
Most loaches do have scales, but they are very small and often sunk into the skin, so the fish looks smooth. Kuhli loaches (Pangio species) and clown loaches (Chromobotia macracanthus) are the classic examples. The same goes for knifefish such as the black ghost (Apteronotus albifrons), the elephantnose (Gnathonemus petersii) and other mormyrids, and spiny eels such as the peacock eel (Macrognathus siamensis). Keepers widely report mormyrids and knifefish as among the first fish to react to medication. Technically these are tiny-scaled, not scaleless, but in practice you treat them as scaleless.
Armoured, not scaled: corydoras and plecos
Corydoras carry two rows of bony plates (scutes) along each side, and plecos are covered in bony plates over most of the body. That armour doesn't stop dissolved chemicals reaching skin and gills, and both groups are widely reported to be sensitive to some treatments, so most labels and keepers group them with scaleless fish. Their barbels and soft bellies are not armoured at all.
Others worth checking
Freshwater puffers lack conventional scales and are usually treated as sensitive, and a few gobies have reduced scale cover, though this varies by species. If you can't find a clear answer for something unusual, assume it is sensitive. Caution with a hardy fish costs little; carelessness with a sensitive one can cost the fish.
Why their skin changes the rules
Fish skin is living tissue under a mucus (slime) coat, and scales sit inside it as an extra layer. In fish with few or tiny scales, the skin is generally more permeable and the slime coat does more of the protective work. In practice:
- They absorb dissolved substances faster. A dose set for the average community fish can hit a loach or catfish much harder.
- Anything that irritates the slime coat hits harder. Harsh chemicals, sudden changes in water chemistry and rough surfaces all damage it.
- Physical damage opens the door to infection. Scratched skin and worn barbels are easy entry points for bacteria and fungus.
The treatments most often flagged as risky are malachite green, formalin, copper, organophosphate parasite treatments and some dyes. That doesn't put every product containing them off limits: many ich treatments with malachite green and formalin carry specific scaleless instructions. It means you read the label first, not after the loaches start gasping.
Medications: how to think differently
Read the label, then follow that label
Before you dose, check the packaging or the manufacturer's website for warnings about scaleless fish, catfish, loaches or invertebrates. Many manufacturers recommend a reduced dose (often half) for these fish; some say the product is unsuitable; some say nothing. When a label gives scaleless guidance, use exactly that. When it doesn't, ask the manufacturer, a vet with fish experience or a specialist retailer first. We deliberately don't give dosing figures for named products, because formulas change between brands and over time.
"Half dose is always safe for scaleless fish. Just halve whatever the bottle says."
A reduced dose is a manufacturer's instruction for a specific formula, not a universal rule. Halving a product with no such guidance may still be too strong for a sensitive species, or too weak to kill the pathogen. Underdosing drags the disease out and can mean repeating the whole course. Follow the specific product's guidance.
Increase gradually and watch the first hours
Where a label gives a range, some keepers start low and build up rather than hitting the tank with a full dose on day one. Either way, stay with the tank for an hour or two after dosing. Rapid breathing, loaches dashing about or lying in the open in daylight, or catfish hanging at the surface mean stop, do a large water change with temperature-matched, dechlorinated water, and add activated carbon.
The hospital tank, scaleless version
A hospital tank lets you treat the sick fish without dosing your display, plants and filter bacteria. For scaleless fish, tweak it: a bare floor is easy to clean but stressful for burrowers like kuhlis, so add a length of pipe or a shallow dish of fine sand. Match the display's temperature and chemistry, and work out the real water volume, because overdosing a small tank is easy. A quarantine period for new arrivals catches problems before they reach the main tank.
Sometimes the display has to be treated, as with ich, which is usually throughout the tank by the time you see spots. Then the most sensitive fish in the tank sets the limit for everyone.
Heat and time for ich, and their limits
Because many ich medications contain malachite green and formalin, loach and catfish keepers often try heat first. Raising the temperature slowly to around 28-30°C speeds up the parasite's life cycle, and many keepers clear mild outbreaks this way; our ich treatment guide covers the timeline. What most guides skip are the limits:
- Not every fish tolerates it. Hillstream loaches and cooler-water corydoras such as the peppered cory (Corydoras paleatus) are poor candidates.
- Warm water holds less oxygen. Add an airstone or make sure the filter outlet breaks the surface.
- It isn't guaranteed. Some ich strains appear to cope with higher temperatures. If new spots keep appearing, switch to a medication labelled for scaleless fish.
- Stopping early as soon as spots vanish is the commonest reason ich comes back.
Never mix treatments
Combining products, or adding a second because the first "isn't working yet", is one of the fastest ways to lose scaleless fish, because the effects can stack unpredictably. Finish one course, then do a large water change and run fresh carbon before starting anything else, unless the manufacturer says two products can be combined.
Formalin uses up dissolved oxygen, and so does a warm tank, so aerate hard if you combine the two. Activated carbon removes most medications: take it out for the course and add fresh carbon afterwards. Copper-based treatments kill shrimp and snails, so keep them away from tanks with invertebrates.
Salt: the honest picture
Salt is often sold as a gentle, "natural" treatment. For scaleless fish it deserves more caution than it gets. Most come from soft, mineral-poor rivers, and many loach and catfish keepers report reactions at salt levels that livebearers shrug off.
Honestly, there are few controlled studies on salt tolerance in the species hobbyists keep, so most advice, ours included, rests on keeper experience and general fish physiology. If you use salt, choose plain aquarium salt (not table salt with additives, not marine salt), start low, raise it gradually and watch closely. Salt doesn't evaporate, so top up with plain dechlorinated water and only replace salt for the water you remove. Many plants dislike it too. If in doubt, leave it out.
Substrate, decor and the abrasion problem
Bare skin and sensitive barbels spend all day against the bottom of your tank. Sharp gravel, lava rock, rough slate edges and ornaments with seams cause small injuries that often become infected, and worn-down barbels are one of the most common problems in corydoras and loaches.
Use smooth sand or small, rounded gravel; our substrate guide covers the options. Run your hand over new rock and wood: if it snags your skin, it will snag theirs. For burrowers like kuhlis and spiny eels, fine sand is how they hide and feel secure.
Water quality: your early-warning system
Because they absorb what is in the water so readily, scaleless fish often show stress before the rest of the tank. Kuhlis wandering in daylight, loaches breathing hard, corydoras dashing to the surface more than usual, or catfish with reddened skin or a cloudy slime coat all mean test ammonia, nitrite, nitrate, pH and temperature. Our water parameters guide explains the numbers.
Give these fish a margin: a mature, fully cycled tank, steady temperature and regular water changes. A tank that just about copes with tetras may be too unstable for loaches.
Handling, netting and acclimation
Nets are rough on bare skin, and many catfish have locking pectoral spines that tangle in mesh. Corydoras, Synodontis and plecos are the usual victims. Where you can, guide the fish into a clear container underwater instead. If you must net, use soft, fine mesh and be patient, and handle spiny catfish carefully, as some can give a painful jab. Kuhlis and spiny eels squeeze through tiny gaps and into filter intakes, so fit a tight lid and cover intakes with sponge.
Acclimate new fish slowly, especially if your water differs from the shop's. Drip acclimation gives sensitive skin and gills time to adjust, and our acclimation guide explains when it is worth the effort.
Tank mates
Avoid persistent fin nippers and boisterous fish that harass anything on the bottom: bare-skinned fish can end up with wounds that never quite heal. Timid fish such as glass catfish and elephantnose do best with calm companions that won't beat them to food. And a peaceful community over sharp gravel still causes damage over time.
Also check that tank mates can cope with the conditions you might need for treatment. Fish that can't take 28°C rule out the heat option for ich before you start. The compatibility checker helps you plan combinations before you buy.
"Scaleless means delicate. These fish are for experts only."
Many scaleless and tiny-scaled fish, from corydoras to kuhlis to bristlenose plecos, are hardy and long-lived in a stable, mature tank with the right substrate. They are less forgiving of chemicals, rough surfaces and sudden changes, not fragile. Get those right and they are often easier than their reputation suggests.
- Confirm the diagnosis. Not every white spot is ich; our fish disease guide helps you narrow it down.
- Test the water first. Poor water quality mimics many diseases, and medicating a tank with an ammonia problem makes things worse.
- Read the label for scaleless and invertebrate warnings. If it says nothing, ask before you dose.
- Work out your real water volume. Substrate and decor mean the tank holds less than its nominal size.
- Remove carbon, add aeration, and have dechlorinated, temperature-matched water ready for an emergency change.
- One treatment at a time, for the full course.
Log ammonia, nitrite, nitrate, pH and temperature through a treatment, keep a timestamped record of water changes in the tank ledger, and check compatibility before adding tank mates with App-aquatic.
Get the free appWhich aquarium fish are scaleless?
Many catfish are truly scaleless, including pictus catfish, Synodontis, glass catfish and banjo catfish. Most loaches, knifefish, elephantnose fish and spiny eels have scales so small or embedded that the hobby treats them as scaleless. Corydoras and plecos have bony plates instead of normal scales and are usually grouped with them for medication purposes. If you are unsure about a species, treat it as sensitive.
Can I use ich medication with loaches in the tank?
Often yes, but only a product that says it is suitable for scaleless fish, used exactly as its label directs. Many ich treatments contain malachite green and formalin, which loaches are known to react badly to. Remove carbon, add aeration and watch the loaches closely for the first few hours. Many keepers try heat first, if every fish in the tank tolerates it.
Is aquarium salt safe for scaleless fish?
It depends on the species and the amount, and the evidence is mostly keeper experience rather than controlled studies. Many loach and catfish keepers avoid salt or use only low levels for short periods, raised gradually. Salt does not evaporate, so it builds up if you top up with salted water, and it harms many plants. If you are unsure, leave it out.
Should I always use a half dose for scaleless fish?
No. Use a reduced dose when the product label tells you to, because the manufacturer has worked that out for that formula. Halving a product with no such guidance is a guess: it may still be too strong, or too weak to kill the pathogen. If the label is silent, ask the manufacturer, a vet or a specialist retailer.
Are corydoras and plecos scaleless?
Not strictly. Corydoras have two rows of bony plates along each side, and plecos are covered in bony plates over most of the body. Neither has normal scales, and both are widely reported to react badly to some medications, so most labels and keepers group them with scaleless fish.
Is heat alone enough to cure ich in a tank with loaches?
Sometimes, but it is not guaranteed. Heat speeds up the parasite's life cycle and many keepers clear mild outbreaks this way, but some ich strains seem to cope with higher temperatures, warm water holds less oxygen, and hillstream loaches and cooler-water corydoras dislike the heat. Raise the temperature slowly, add aeration, and switch to a scaleless-safe medication if spots keep appearing.
All guides · Ich treatment · Hospital tank · Loach types · Kuhli loach care · Corydoras care · Bristlenose pleco care · Get the app