Dropsy in fish: pinecone scales, causes and treatment
Swollen body, scales raised like a pinecone: what dropsy means, the Epsom salt and antibiotic plan that can save an early case, and when to stop.
By Xavier Jodoin
In short
Dropsy is not a disease but fluid building up inside a fish whose kidneys have stopped coping, usually from a bacterial infection that took hold in poor water. Move the fish to a hospital tank, add Epsom salt at one tablespoon per 40 litres, feed an antibiotic in food if it still eats, and fix the water in the main tank. Caught before the scales stand out it is often survivable; a fully pineconed fish that has stopped eating rarely is.
What you are looking at
The body is swollen, not just the belly, and looked at from above the scales stand out from the body like the scales of a pinecone. That silhouette is the sign, and it is what separates dropsy from a fish that is simply overfed or carrying eggs: a fat fish is smooth, a dropsied fish is bristled. Alongside it you will usually see one or both eyes pushed outward, pale or red gills, a fish that hangs near the surface or sits on the bottom, long pale stringy faeces, and a loss of appetite that arrives a day or two before the swelling does.
Dropsy is not a disease. It is a description — fluid accumulating in the body cavity and under the skin — and it means the kidneys have stopped keeping the fish’s internal water balance. A freshwater fish lives in water far more dilute than its blood, so water pours in through the gills and skin all day and the kidneys spend all day pumping it out. When they fail, the fish fills up. What stops the kidneys is nearly always a bacterial infection, most often Aeromonas, which lives in every aquarium and is harmless to a healthy fish. It got in because the fish was already run down: weeks of nitrate creeping up, a spell of ammonia after a missed water change, a temperature that was too low for too long, old age, or the stress of shipping and a new tank.
That is why dropsy so often appears in one fish while its tankmates look fine, and why it is rarely “caught” from another fish. The bacteria were always there. The water is what changed.
The honest prognosis
Once the scales are fully out, most fish do not recover, whatever you do, because the organ damage behind the swelling is already done. Caught early — a fish that is a little swollen, off its food, with scales only just starting to lift along the back — the odds are real, perhaps half. The treatment below is worth doing in the early case and worth trying in the late one, but go in knowing the difference, so that a fish that does not improve within a week is not kept suffering for a fourth.
The treatment
1. Move the fish. A bare hospital tank of 10 to 20 litres with a heater, an air stone and a sponge filter. Not because the dropsy will spread — it will not, in the usual sense — but because every step below is easier and cheaper in a small volume, and the fish is spared the attention of tankmates that will pick at a weak one.
2. Epsom salt, not aquarium salt. Magnesium sulphate at one tablespoon per 40 litres (10 gallons), dissolved in a cup of tank water first. It draws fluid out through the skin and gives the kidneys a chance to catch up, and it is the one part of the treatment that visibly helps within a day or two. Aquarium salt (sodium chloride) does the opposite — it adds to the salt load the fish is already failing to manage — so do not substitute it.
3. Warmth, gently. For tropical fish, 27–28 °C, raised by no more than 1 °C every twelve hours. Warmth speeds the immune system and the antibiotic both. A goldfish stays at its normal temperature; do not heat a cold-water fish to treat it.
4. An antibiotic, ideally in the food. The infection is internal, and medication dissolved in the water reaches it poorly through a fish that is not drinking. If the fish is still eating, a medicated food — a kanamycin- or metronidazole-based product, or pellets soaked in the medication — puts the antibiotic where the bacteria are. If it has stopped eating, dose the water with a broad-spectrum antibacterial (kanamycin, or a nitrofurazone product) for the hospital tank’s actual volume, carbon removed, and complete the course on the label even if the fish looks better on day four.
5. Daily water changes in the hospital tank, 25–50 %, re-dosing the salt and the medication for the volume you replace. Clean water is half of every fish treatment, and here it is the half that lets the other half work.
In the app — a checkup with a photo from above and the side, your readings and the symptoms ticked gives Finley the severity and a plan sized to it; Add plan to my schedule books the daily changes, the salt and the medication days as tasks, and the follow-up asks whether the swelling is going down.
The main tank, meanwhile
The fish in the hospital tank is the symptom. The main tank is the cause, and if you only treat the one, the next run-down fish in the community is the next case. Test ammonia, nitrite, nitrate, pH and temperature today. Anything above zero for the toxins or nitrate over 40 ppm gets a 30–50 % water change now and daily testing until it clears; nitrate that has been sitting in the 20s and 30s for months gets a proper weekly schedule from this week on. Vacuum the substrate while the water is out, because decaying food in the gravel is where Aeromonas multiplies, and look for what nobody noticed — a dead snail behind the filter, a heater reading two degrees low, a fortnight of double feeding.
In the app — log the test and the verdict says which readings are the problem and by how much; the trend chart shows whether nitrate has been climbing for weeks, which is the usual story behind a dropsy case.
Knowing when to stop
A fish that is improving eats again, the swelling eases, and the scales lie back down over a week or two. A fish that is not — fully pineconed, not eating, breathing hard, unable to hold itself upright — is not going to, and another week of Epsom salt is another week of suffering. Euthanasia with clove oil (a few drops in a small container of tank water, shaken to emulsify, added slowly until the fish is asleep, then a strong dose) is quick and painless, and is the last kindness a keeper can do. It is a hard call and an honest one, and the guide to recognising suffering applies here too.
Common questions
What does dropsy look like in fish?
A swollen body, not just a round belly, with the scales standing out from it like a pinecone when you look from above. Bulging eyes, pale gills, stringy pale faeces and a fish that stops eating a day or two before the swelling are the usual companions. A fat or egg-bound fish is smooth; a dropsied fish is bristled.
Is dropsy contagious?
Not in the way ich is. The bacteria usually behind it live in every aquarium and only overwhelm a fish that is already weakened, almost always by poor water. A second case in the same tank is not catching it from the first; it is living in the same water. Treat the tank’s chemistry for everyone and isolate the sick fish for its own sake.
Can a fish recover from dropsy?
Sometimes, if it is caught early — a fish that is a little swollen and off its food, with scales only beginning to lift, has perhaps even odds with Epsom salt, warmth, an antibiotic and clean water. A fish whose scales stand fully out and that has stopped eating rarely recovers, because the organ damage behind the swelling is already done.
Does Epsom salt cure dropsy?
It helps. Magnesium sulphate at one tablespoon per 40 litres draws fluid out through the skin and takes pressure off the kidneys, which is why it is the one step that visibly helps within a day or two. It does not treat the infection underneath, which needs an antibiotic, ideally in the food. Aquarium salt is not a substitute and makes things worse.
What causes dropsy in the first place?
A bacterial infection, usually Aeromonas, that reaches the kidneys of a fish whose defences were down. What lowered them is nearly always the water: nitrate creeping up for weeks, ammonia or nitrite after a missed water change, a temperature too low for too long, or the stress of shipping into a new tank. Age and overfeeding add to it. Fix the water and most tanks never see a second case.