Bloat in fish is treatable, but the right approach depends on what’s causing the swelling. The most common causes are constipation from overfeeding, internal bacterial infections, and parasites. Simple digestive bloat can resolve in a few days with fasting and Epsom salt, while infection-driven bloat requires medicated food. Acting quickly matters: the longer bloat goes untreated, the more likely it is to cause permanent organ damage.
Identify the Type of Bloat First
Before you treat anything, look closely at your fish. Not all swelling is the same condition, and treating for the wrong one wastes critical time.
Constipation bloat is the most common and least dangerous form. The fish looks swollen in the belly, may have reduced appetite, and often produces little or no waste. It’s typically caused by overfeeding, low-fiber diets, or dried foods that expand in the gut. This is especially common in bettas and goldfish.
Infection-driven bloat (often called Malawi bloat in cichlids) involves intestinal bacterial infections or internal parasites. Roughly 70% of these cases involve bacterial infections, and about 40% involve a specific intestinal parasite. Fish with this type of bloat often have white, stringy feces, refuse food entirely, and may become lethargic or hide. African cichlids fed too much protein or fat are particularly vulnerable.
Dropsy looks like bloat but is a different condition entirely. The key visual difference: in dropsy, the fish’s scales push outward from the body, creating a “pinecone” appearance when viewed from above. This happens because fluid is accumulating in the abdominal cavity, often due to kidney failure or a blocked kidney drainage tube. If your fish has raised scales, you’re dealing with dropsy, not standard bloat.
Treating Constipation Bloat
If your fish is swollen but still somewhat active, has no pineconing scales, and was recently eating a heavy or low-fiber diet, constipation is the most likely cause. Start with the simplest intervention first.
Stop feeding the fish for 3 to 7 days. This alone resolves many cases. Fish can go without food much longer than most people expect, and fasting gives the digestive tract time to clear whatever is backed up. After the fasting period, reintroduce food gradually with high-roughage options like frozen daphnia, frozen cyclops, baby brine shrimp, or blanched peas (skinned and mashed).
For stubborn cases, an Epsom salt bath speeds things along. Epsom salt (magnesium sulfate) acts as a gentle laxative and draws excess fluid out of swollen tissue. The recommended dose is about 15 grams of Epsom salt per 20 liters of water (roughly 1 tablespoon per 5 gallons). Set up a separate container with tank water at the same temperature, dissolve the salt completely, and keep the fish in this bath for 2 to 3 days. Severe cases may need up to 5 or 6 days. Watch your fish closely during treatment and return it to clean tank water if it shows signs of distress.
Treating Infection-Driven Bloat
If fasting doesn’t help after a few days, or if the fish has white stringy feces, complete loss of appetite, or rapid decline, you’re likely dealing with an internal infection or parasite. This is where many fishkeepers make a costly mistake: adding medication to the water column. Internal diseases like bloat can only be effectively treated through medicated food, because the infection lives inside the gut. Dissolving medication in the tank water does not deliver enough of the active ingredient to the intestines to make a difference.
The key medication for most bloat infections is metronidazole, which targets both intestinal bacteria and common gut parasites. You can find it in several over-the-counter aquarium products: SeaChem MetroPlex, API General Cure (which contains 250 mg of metronidazole per packet along with a deworming agent), Hikari Metro Plus food, and New Life Spectrum Hex-Shield food. The last two are pre-medicated foods, which makes dosing simpler.
If you’re using a powdered medication like MetroPlex, you’ll need to mix it into the food yourself. The easiest method is to make a paste by mixing a small amount of food with tank water, then adding roughly 1/16 teaspoon of medication per portion. Some fishkeepers use gel-based foods or garlic-soaked pellets to make the medicated food more appealing, since sick fish are often reluctant to eat. If your fish refuses food entirely, you may need to try adding the medication to the water as a last resort, understanding that it’s far less effective for internal conditions.
For cases involving both bacterial infection and parasites, combining metronidazole with a broad-spectrum antibiotic in the food covers both fronts. Treatment typically lasts 7 to 10 days, and you should see improvement within the first few days if the medication is reaching the gut.
Bloat in Bettas and Swim Bladder Problems
Bettas are particularly prone to digestive bloat because of their small stomachs and the tendency of owners to overfeed them. A bloated betta often develops swim bladder problems as a secondary effect: the swollen gut presses against the swim bladder, causing the fish to float sideways, sink to the bottom, or bob helplessly at the surface.
The good news is that treating the bloat usually fixes the swim bladder issue too. Fast the betta for 3 to 5 days, then offer a small piece of blanched, skinned pea. If the bloating doesn’t resolve, move to an Epsom salt bath at the standard dose. Going forward, feed bettas only as much as they can eat in about two minutes, once or twice a day. A single fasting day per week helps prevent recurring problems.
When Bloat Is Actually Dropsy
If you notice scales standing away from the body like a pinecone, the situation is more serious. Dropsy indicates that an internal organ, usually the kidney, has swollen and is leaking fluid into the abdominal cavity. By the time scales are visibly raised, the condition is advanced.
Treatment involves adding a broad-spectrum antibiotic to the food in a hospital tank (a small, separate aquarium). Recovery from dropsy is possible but depends heavily on the underlying cause and how early you catch it. Fish that respond to treatment may recover within a few days to a few weeks. Unfortunately, many cases of advanced pinecone dropsy do not respond to treatment.
Preventing Bloat From Coming Back
Most bloat is preventable with the right feeding habits and water quality. The single biggest cause is overfeeding. Fish stomachs are tiny, often roughly the size of their eye, and dried foods expand significantly after being swallowed. Feed only what your fish can consume in two to three minutes, and remove uneaten food promptly.
Building a fasting day into your weekly routine gives the digestive system a regular reset. One day per week with no food is a simple, effective prevention strategy. For species like African cichlids, diet composition matters as much as quantity. These fish need vegetation-heavy diets. Feeding them high-protein or high-fat foods designed for other species is one of the most common triggers for Malawi bloat.
Keep your water clean and your filtration working well. Poor water quality stresses fish, weakens their immune systems, and makes them vulnerable to the opportunistic bacteria and parasites that cause infection-driven bloat. Regular water changes and avoiding overstocking go a long way toward keeping bloat out of your tank entirely.