Disease happens. Even in well-maintained tanks with stable water chemistry and carefully sourced livestock, pathogens can take hold — often triggered by stress, a sudden temperature drop, or a new fish introduced without quarantine. The difference between losing one fish and losing an entire system usually comes down to how quickly and accurately you identify what you’re dealing with, and whether you have the right tools ready before the problem escalates.
This guide covers the four diseases that account for the vast majority of calls and messages we receive: White Spot (Ichthyophthirius multifiliis), Fin Rot (Aeromonas / Pseudomonas spp.), Velvet (Oodinium / Piscinoodinium), and Columnaris (Flavobacterium columnare). For each one: what it looks like, why it spreads, what actually works in the UK, and — most importantly — how to keep it out in the first place.
Small white pustules, 0.5–1 mm each, scattered across fins and body. Under good light they look exactly like grains of salt. Affected fish scratch against decor (flashing), breathe faster than normal, and in heavy infections clamp their fins. Ich is a ciliate protozoan with a three-stage lifecycle: the trophont (the white cyst feeding under the skin), the tomont (the encysted, dividing stage that falls to the substrate), and the free-swimming theront (the infective stage that must find a host within 24–48 hours or die).
The critical point: you cannot kill the parasite while it is inside the cyst on the fish. Treatment only kills the free-swimming theront. This means treatment must run long enough — typically 7–10 days at tropical temperatures — to catch every reproductive cycle.
| Product | Active ingredient | Notes |
|---|---|---|
| eSHa EXIT | Formaldehyde + malachite green | Widely available, effective, won’t crash your filter at recommended dose |
| Waterlife Protozin | Malachite green + acriflavine | Long-established UK product; follow day 1/2/6 dosing schedule precisely |
| NT Labs Anti-Ulcer & Finrot | — | Not ich-specific; see Fin Rot section |
| Aquarium salt (NaCl) | Sodium chloride | 1–3 g/litre adjunct; reduces osmotic stress, not a standalone cure |
Temperature tip: Raising tank temperature to 28–30 °C accelerates the lifecycle, pushing more theronts into the water simultaneously and shortening the treatment window to 5–7 days. Do this gradually (max 1 °C per hour) and increase surface agitation because warmer water holds less oxygen. Do not do this with scaleless species — Bichirs (Polypterus spp.), Rays (Potamotrygon spp.), Corydoras, and L-number Plecos are all more sensitive to both elevated temperature and malachite green. Use half-dose and a longer treatment period for these groups.
Off-the-shelf “natural” alternatives based on aloe or botanical extracts have no peer-reviewed evidence of efficacy against I. multifiliis. Don’t waste time on them when a £6 bottle of eSHa EXIT exists.
Fin edges that appear ragged, discoloured (white, grey, or red-edged), and progressively receding toward the body. In early-stage bacterial fin rot the fin membrane between rays deteriorates first; in advanced cases the rays themselves are affected and the rot can spread to the body, at which point the infection is systemic and significantly harder to treat. It’s also worth noting that fin rot is almost always secondary — the primary cause is usually water quality, physical damage, or stress. Fix the cause first, or treatment will fail.
| Product | Active ingredient | Notes |
|---|---|---|
| NT Labs Anti-Ulcer & Finrot | Acriflavine + phenoxyethanol | Treats external bacterial infections including Columnaris; gentle enough for scaleless fish at correct dose |
| eSHa 2000 | Proflavine + acriflavine + copper sulphate | Broad-spectrum; effective against gram-negative bacteria |
| API Melafix | Tea tree oil (Melaleuca) | Mild; useful for very early-stage damage and post-treatment healing, not for active moderate-severe infections |
| Salt bath | NaCl | 5 g/litre short-bath (5 min) for large fish that are hard to treat in-tank |
Water quality first: Before you dose anything, test nitrate, nitrite, ammonia, and pH. A 30–40% water change with dechlorinated, temperature-matched water costs nothing and often visibly improves a fish before medication arrives. Fin rot will recur until the underlying environment is corrected. Target: ammonia 0 ppm, nitrite 0 ppm, nitrate below 20–40 ppm depending on species.
Velvet is routinely misdiagnosed as Ich. The giveaway: the cysts are much smaller (0.1 mm vs. 0.5–1 mm), giving the fish a dusty or velvety gold-to-rust sheen rather than salt-grain spots. Best viewed with a torch held at a low angle to the fish. Affected fish often show rapid gill movement first, before visible skin changes, because Oodinium preferentially attacks gill tissue. This makes it more dangerous than Ich — you can lose fish quickly from respiratory failure before the skin presentation is obvious.
Like Ich, Oodinium is a three-stage parasite. Unlike Ich, the tomont stage (substrate cyst) can persist for weeks, and the dinospore (free-swimming stage) can photosynthesize. Reducing light to near-zero during treatment eliminates the dinospore’s energy source and improves treatment outcomes meaningfully.
| Product | Active ingredient | Notes |
|---|---|---|
| eSHa EXIT | Formaldehyde + malachite green | Effective against both Ich and Velvet; run for full 14 days for Velvet |
| Waterlife Protozin | Malachite green + acriflavine | Label covers Velvet specifically; follow extended dosing schedule |
| Copper-based treatments (e.g. Seachem Cupramine) | Copper sulphate | Highly effective but lethal to invertebrates and scaleless species at any dose; not recommended for mixed community tanks |
Light blackout: Cover the tank on all sides for the duration of treatment. Dim ambient room light where possible. This is not optional — it’s the single most effective non-chemical intervention for Velvet.
Raise temperature to 28 °C (same caveats as Ich) to accelerate lifecycle.
Columnaris is a gram-negative bacterium that presents in several ways, which is why it’s frequently misidentified:
Columnaris thrives in warmer, harder water and high organic loads. It’s commonly introduced via new fish, nets, or hands that have been in an infected tank. It is not the same as fungal infection (Saprolegnia spp.), though the two can occur simultaneously.
| Product | Active ingredient | Notes |
|---|---|---|
| NT Labs Anti-Ulcer & Finrot | Acriflavine + phenoxyethanol | Front-line UK option; targets gram-negative bacteria |
| eSHa 2000 | Proflavine + acriflavine | Covers both bacterial and fungal co-infections |
| Sera Baktopur | Ethacridine lactate | Available from specialist retailers; effective against Flavobacterium |
| Furan-2 (API) | Nitrofurazone + furazolidone | Prescription-free in the UK; stronger option for acute cases; remove carbon from filter |
Lower temperature slightly (to 24–25 °C if the species allows) — this slows the acute strain significantly. Do not use salt as a standalone treatment; Flavobacterium columnare can be salt-tolerant at concentrations that are safe for fish.
Every disease guide eventually says “quarantine new fish.” Most fishkeepers skip it. Here’s the actual protocol, not a vague suggestion:
| Day | Action |
|---|---|
| 0–3 | Observe only. No medication unless symptoms are present. Let the fish settle and show normal behaviour. |
| 3–7 | Begin prophylactic treatment if fish came from a source with unknown health history. eSHa EXIT for protozoans is a reasonable precaution. |
| 7–14 | Continue observation. Check for delayed presentation of Columnaris or Velvet (both can be latent). |
| 14–21 | If the fish is eating, active, and showing no symptoms, it is safe to transfer. |
| 21+ | Extended hold for wild-caught specimens (30 days minimum). Wild-caught fish from South East Asia and Indonesia may carry internal parasites not visible externally — consider a broad-spectrum dewormer (e.g. Kusuri Wormer Plus) as standard. |
We hold all fish at MTF for a minimum quarantine period before dispatch. Wild-caught stock — Corydoras (CW and C-number species), L-number Plecos, and larger predatory species — gets longer. That’s not marketing copy; it’s the reason we have a Live Arrival Guarantee we’re confident standing behind. Read more about how we source and hold livestock →
Ninety percent of disease outbreaks have a water quality event in their history. These are the parameters worth maintaining with genuine precision:
| Parameter | Target range | Why it matters for disease |
|---|---|---|
| Temperature | Species-specific ±0.5 °C | Temperature swings of 2–3 °C suppress immune function |
| Ammonia | 0 ppm | Any detectable ammonia damages gill epithelia, creating entry points for pathogens |
| Nitrite | 0 ppm | Nitrite binds haemoglobin, causing immune suppression |
| Nitrate | <20–40 ppm | Chronically elevated nitrate correlates with increased bacterial infection rates |
| pH | Species-specific, stable | Swings (even within acceptable range) cause stress |
| Dissolved oxygen | >6 mg/litre | Low O₂ favours anaerobic pathogens and weakens fish response |
| TDS/conductivity | Species-specific | Incorrect conductivity stresses osmoregulation, a known Columnaris risk factor |
This point cannot be overstated for the fish MTF customers tend to keep. Bichirs (Polypterus spp.), Rays (Potamotrygon spp.), Pangasiid catfish, Corydoras, Pictus catfish, and all L-number Plecos are significantly more sensitive to copper-based treatments and malachite green than scaled fish. Standard doses that are safe for a cichlid can be fatal for a Polypterus.
General rule: Use half the label dose for scaleless species, maintain high oxygenation throughout treatment, and be prepared to do a 30–50% water change if you see any signs of distress.
For Rays (Potamotrygon spp.) specifically: most chemical treatments are high-risk. Salt therapy (1–2 g/litre, not higher) and temperature management are safer first-line approaches. If you’re keeping Stingrays and suspect disease, seek specialist advice before medicating.
| Symptom | Most likely cause | Secondary possibility |
|---|---|---|
| Salt-grain white spots on fins/body | Ich | Lymphocystis (larger, cauliflower-like) |
| Fine gold/rust dust, gill flaring | Velvet | Ich (coarser texture) |
| Ragged fin edges, white margins | Fin Rot | Physical damage |
| Saddle lesion behind dorsal, fuzzy patches | Columnaris | Fungal (Saprolegnia) |
| Cottony mouth, lethargy | Columnaris | Fungal |
| Rapid gill movement, no visible lesions | Velvet (gill form) | Bacterial gill disease |
| Flashing, no visible spots | Early Ich or Velvet | Skin flukes (Gyrodactylus) |
The worst time to research medication is at 10 pm with a tank full of sick fish. Keep a basic disease kit assembled:
If you’re keeping wild-caught stock — wild Corydoras, L-numbers from the Rio Tapajós, or any fish that has spent time in a transhipping bag — add Kusuri Wormer Plus (flubendazole) to the kit for internal parasites.
Disease diagnosis is not complicated once you know what you’re looking for. The protocols above are not over-cautious — they reflect what works and what is safe for the fish that MTF customers actually keep, including the large scaleless species where standard dosing can kill the patient faster than the pathogen.
Buy healthy fish from a supplier who quarantines before dispatch, test your water regularly, and keep a stocked disease kit. Those three things will prevent most of what this guide treats.
Browse our current stock — every fish ships with our Live Arrival Guarantee and has been health-checked and quarantined before dispatch. View available livestock and current auctions →
