Why Is My Fish Swimming Sideways? UK Aquarist's Guide

Quick action plan

If your fish is swimming sideways or upside down RIGHT NOW:

1. Stop feeding immediately — fast for 3 days
2. Test water (ammonia, nitrite, nitrate, pH, temperature) — fix any issues first
3. Raise temperature to 27-28 °C if not already
4. Add 1 tablespoon Epsom salt per 20 L of water (NOT aquarium salt — Epsom is magnesium sulfate)
5. After 3 days fasting: feed 1-2 shelled cooked peas per fish (skinned, broken into bits)
6. Continue peas + Epsom for 5-7 more days
7. If no improvement after 10 days: switch to antibacterial treatment

Most constipation-related cases improve within a week.

What is the swim bladder?

The swim bladder is a gas-filled internal organ that controls a fish's buoyancy — like an inflatable balloon inside the body. When working properly, the fish stays effortlessly at any depth.

When the swim bladder fails to inflate correctly:

• Too inflated → fish floats at surface or upside down
• Not inflated → fish sinks to bottom and can't rise
• Asymmetrically inflated → fish swims sideways or in circles

Swim bladder disorder isn't really a disease — it's a SYMPTOM. The actual cause varies.

Common causes

1. Overfeeding / constipation (most common — 60% of cases)

Pellets that expand in the stomach press on the swim bladder. Common in:

• Bettas fed too many pellets
• Goldfish (especially fancy varieties)
• Balloon mollies (compressed body shape)

Symptoms: Fish swims sideways AFTER feeding, sometimes recovers between meals. Bloated belly visible.

Treatment: Fasting + peas + Epsom salt (see action plan above).

2. Bacterial infection (20% of cases)

Internal bacterial infection inflames the swim bladder.

Symptoms: Persistent sideways swimming for 7+ days. Loss of appetite. Sometimes red/inflamed anal area. Possible secondary symptoms (fin rot, popeye).

Treatment: Antibacterial medication (Esha 2000 has antibacterial action; for severe cases, Maracyn or Kanaplex if available). Consult an aquatic vet for serious cases.

3. Cold stress (10% of cases)

Cold water slows fish digestion and metabolism. Common after winter heater failure.

Symptoms: Sluggish movement, occasional sideways drifting. Often combined with clamped fins.

Treatment: Raise temperature to 26 °C gradually. Resume feeding only after fish behavior normalises.

4. Genetic / congenital (5% of cases — terminal)

Fancy goldfish (oranda, ranchu, telescope eye), balloon mollies, balloon rams — body modifications affect the swim bladder permanently.

Treatment: None. Manage symptoms with shallower water and frequent small feedings. Some keepers use floating "swim bladder slings" — DIY foam slings to support the fish in normal orientation.

5. Poor water quality (5% of cases)

High ammonia/nitrite damages internal organs including the swim bladder.

Treatment: Large water changes immediately. Test and fix root cause.

The pea treatment (detailed)

Peas have been used for decades as a natural laxative for fish. Here's how:

1. Get frozen or fresh shelled peas (NOT the canned salty kind)
2. Microwave 1-2 peas for 15 seconds in water
3. Squeeze the pea out of the skin — discard skin
4. Mash the pea inside into 4-6 small pieces
5. Drop pieces into the tank — fish should eat them within minutes
6. Repeat once daily for 3-5 days alongside fasting from regular food

The high fibre + the soft texture helps push blockages through the digestive tract, relieving pressure on the swim bladder.

Epsom salt treatment

Epsom salt = magnesium sulfate (MgSO4). NOT the same as aquarium/sea salt. Epsom acts as a laxative.

Dose: 1 tablespoon (15 g) per 20 litres of tank water.

How:

1. Pre-dissolve in a jug of warm tank water
2. Add gradually over 30 minutes
3. Maintain for 5-7 days
4. After treatment: do gradual water changes over a week to dilute

Safe for: most tropical fish including bettas, goldfish, mollies, plecos.

Use caution with: very small/sensitive fish (chili rasboras, ember tetras) — use half dose.

What if it doesn't improve?

After 10 days of fasting + peas + Epsom + warm water, if the fish is still swimming sideways:

1. Likely cause: bacterial infection — switch to antibacterial medication
2. Or genetic — accept it's permanent for fancy goldfish/balloon body shapes
3. Consider euthanasia for fish that can no longer eat or swim (clove oil method is humane — search "clove oil fish euthanasia uk")

Prevention

90% of swim bladder cases come from overfeeding. Prevent by:

1. Feed once daily, not multiple times — fish digest better with one meal
2. Pinch only what fish eat in 30 seconds — anything more is too much
3. Soak pellets before feeding — pre-expanded pellets won't bloat in the stomach
4. Add fibre to diet — daphnia, frozen brine shrimp, occasional vegetable matter
5. One fast day per week — let the digestive system clear
6. Don't keep fancy goldfish in tropical tanks — wrong temperature, body shape predisposes

Common mistakes

Mistake 1: Feeding more food because "fish is hungry"

Why it fails: Constipated fish that can't swim properly LOOK hungry but feeding worsens the problem.

Fix: Fast for 3 days first. Hunger is fine for fish — they survive 7+ days without food.

Mistake 2: Adding aquarium salt instead of Epsom

Why it fails: Aquarium salt (sodium chloride) doesn't help digestion. Epsom (magnesium sulfate) does.

Fix: Use Epsom salt only — buy from chemist or pet shop.

Mistake 3: Treating constipation as bacterial infection from day 1

Why it fails: Antibiotics are stressful and unnecessary for simple constipation. Wasted dose and possible resistance buildup.

Fix: Try fasting + peas + Epsom for 7 days FIRST. Only escalate to antibiotics if no improvement.

Mistake 4: Ignoring the underlying cause

Why it fails: Cure the symptom, fish gets sick again next month from same cause.

Fix: Identify what triggered it (overfeeding? cold? bad water?) and fix that long-term.

Summary

Fish swimming sideways = swim bladder disorder, usually from overfeeding. Try fasting + peas + Epsom salt + warm water for 7-10 days. Most cases recover. If not, suspect bacterial infection. Prevent by feeding less, less often, with fibre.

Related guides

• Tropical Fish Tank Temperature
• Ich (White Spot) Treatment
• Fin Rot Treatment Guide
• Fish Tank Nitrogen Cycle
• Browse all freshwater fish