Swim Bladder Disorder: Why Your Fish Floats, Sinks or Swims Sideways

Few things worry a new fishkeeper more than watching a fish that cannot stay level. It bobs up to the surface and cannot swim down, or it lies on its side on the substrate, or it swims nose-down in slow circles. Nine times out of ten what you are looking at is a swim bladder problem, and the good news is that the most common cause is something you can fix at home and, better still, prevent. I have watched a fair few of my own fish go through this, mostly greedy livebearers that ate too much, and the pattern is usually the same.

The honest first thing to say is that “swim bladder disorder” is a description of a symptom, not a single disease. The fish has lost control of its buoyancy. Why it has lost that control is the real question, and the answer changes what you should do.

What the swim bladder does

The swim bladder is a gas-filled organ that sits in the body cavity and lets a fish hold its position in the water without constantly swimming. The fish adjusts the gas inside it to hover, rise or sink, a bit like a diver’s buoyancy vest. When it works, you never think about it. When it stops working, the fish either becomes too buoyant and floats, or loses lift and sinks, or fills unevenly and rolls onto its side.

There is one distinction worth understanding, because it explains a lot. Some fish, including goldfish, danios, tetras and most of the livebearers I keep, have a small duct connecting the swim bladder to the gut. These fish can gulp air at the surface and burp out gas to adjust, and their swim bladder is easily squashed by a swollen, overfull digestive tract sitting right next to it. Other fish, many cichlids among them, have no such duct and regulate the gas chemically through the blood. This is partly why the classic “floating goldfish” is such a cliché: the fish most people keep are exactly the ones whose guts can press on the swim bladder.

The most common cause: overfeeding and constipation

In a home aquarium, by far the most frequent trigger is diet. A fish that is overfed, or fed nothing but dry flake and pellet that swells in the gut, becomes constipated. The blocked, gas-filled intestine presses against the swim bladder and throws the buoyancy out. This is the case I see again and again, and it is the one with the best outcome.

The tell is a fish that was fine, has a slightly swollen belly, and starts floating or struggling to stay down not long after feeding, often a fish that always shoves to the front at feeding time. Dry foods are a particular culprit because pellets and flake absorb water and expand once eaten. My own worst offenders have always been mollies and platies, which will eat until they can barely swim if you let them.

Because this is the common cause, it is also the first thing to rule out, and the treatment is gentle enough that trying it does no harm even if you are wrong.

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How I treat a constipated fish

The routine is simple and it is mostly about giving the gut a chance to clear.

1. Stop feeding. Fast the whole tank for 24 to 72 hours. A healthy fish comes to no harm from a few days without food, and fasting lets a backed-up gut empty and takes the pressure off the swim bladder. For a mild case this alone often does it.
2. Then offer a pea. After the fast, feed a small piece of cooked, shelled pea. Boil a frozen or fresh garden pea for a minute until soft, cool it, slip off the skin, and mash or cut a piece to a size the fish can eat. Peas act as a mild laxative and the fibre helps shift the blockage. This works for omnivores like goldfish, bettas and livebearers; it is not the right food for strict carnivores, whose guts are not built for plant matter.
3. Warmth and calm. Keeping the tank at the upper end of the species’ comfortable range can help digestion along. Turning the lights down and reducing flow lets a struggling fish rest instead of fighting the current.
4. Check the water while you are at it. Poor water quality stresses fish and can mimic or worsen buoyancy trouble, so test and do a water change if the numbers are off. My guide to aquarium water parameters covers what to look for.

A dietary case usually shows improvement within two to four days, and up to a week for a stubborn one. Once the fish is right, fix the cause: feed less, feed smaller amounts more often rather than one big meal, soak dry food before feeding, and work some variety in. My piece on feeding aquarium fish sets out the amounts I actually use, and the short version is that almost everyone overfeeds.

When it is not constipation

If fasting and peas do nothing, and especially if there are other symptoms, you are probably not dealing with a simple blockage. The other causes are harder to treat and some cannot be fixed at all.

• Bacterial infection. An internal infection can inflame or damage the swim bladder. Look for clamped fins, loss of appetite, redness, or the pinecone-like raised scales of dropsy, which is a serious sign. Infection-driven cases sometimes respond to an aquarium-safe antibacterial treatment, but the outlook is guarded, and getting the water spotless matters as much as any medication. My notes on fin rot and on why fish die cover the wider picture of a sick fish.
• Poor water quality. Ammonia or nitrite in the water, or a tank that was never properly cycled, stresses the whole fish and can show up as odd swimming among other signs. This is one more reason a new tank problem so often traces back to the nitrogen cycle.
• Physical injury. A fish that jumped, was netted roughly or was bullied can damage the organ. There is not much to do but keep the water clean and the tank calm and hope it recovers.
• Congenital deformity. Some fish, fancy goldfish especially, are bred into short, round bodies that crowd the internal organs, and a few are simply built to have buoyancy trouble for life. Heavily inbred fancy bettas can be the same. In these cases there is no cure, only management.

The chronic case: living with it

Not every swim bladder fish gets better, and it is worth being honest about that rather than chasing cure after cure. A fish with a permanent buoyancy problem can still have a decent life if it can reach food and is not being harassed. Lowering the water level reduces the effort of getting up and down. Gentle flow, a quiet tank and easy-to-reach food all help. I have kept the odd permanently wonky fish that ate well and lived a normal span, tilted the whole time.

But there is a line. A fish that cannot eat, is being battered against equipment by its own floating, or is clearly suffering with no improvement over weeks is a fish whose kindest option is a humane end. That is never a nice call to make, and it is part of keeping animals honestly.

Preventing it in the first place

Most swim bladder trouble I have seen was avoidable, and the prevention is dull but effective.

• Feed less than you think. Small amounts, once or twice a day, only what is eaten in a couple of minutes. A weekly fasting day does no harm and I use one.
• Vary the diet and add fibre. Not just dry flake. Frozen foods, the occasional blanched vegetable for the omnivores, and quality as the base.
• Soak dry food. A few seconds in tank water before feeding stops pellets swelling inside the fish.
• Keep the water right. A cycled, clean, stable tank prevents a long list of problems, this among them. A steady maintenance routine is the single best insurance.
• Choose sensibly bred fish. If you want to avoid the built-in version of this, be wary of the most extreme fancy goldfish and balloon-bodied shapes; their form is the problem.

Get the feeding and the water right and swim bladder disorder becomes a rare visitor rather than a regular one. When it does turn up, start with the fast and the pea, watch closely, and let the fish’s other symptoms tell you whether you are dealing with a full belly or something more serious.