Glass surfing: why fish swim up and down the glass or stay at the top of the tank

Glass surfing is when a fish swims up and down, or back and forth, against the glass over and over. It is usually a stress sign, and the most common hidden cause is poor water, so test ammonia, nitrite and nitrate first. If the water is fine, look at tank size, reflections, bullying and hiding places.

A fish that stays at the very top of the tank is a related but more urgent warning: it may be short of oxygen. If fish are gulping at the surface, go to gasping fish now.

How to identify it

Glass surfing: the fish swims with its nose or side against the glass, rising and falling in the same spot for minutes or hours, often in one corner.
Staying at the top: the fish hangs just under the surface, near the filter outlet, or at an angle with its mouth near the air.
Gasping: fast gill movement and mouth breaking the surface. This is an oxygen or water-quality emergency, not glass surfing.

Note what else you see: clamped fins, scratching on decor (flashing), torn fins, spots, or one fish chasing others. These point to the cause.

When it's a problem

Normal: new fish surfing for the first one to three days; fish rushing the front glass when you walk past (begging for food); schooling fish such as pygmy corydoras or rasboras swimming with their reflection, which can be harmless.
Early problem: one fish surfing for more than a few days, otherwise eating and looking healthy.
Serious: surfing plus clamped fins, poor appetite, scratching, or several fish doing it at once.
Emergency: fish at the surface gasping, or lying on the bottom. Test water and change water now.

What to do right now

Test ammonia, nitrite, nitrate, pH and temperature. Any ammonia or nitrite is a problem (see ammonia and nitrite).
If anything is off, do a 30–50% water change with dechlorinated water at the same temperature.
Check the filter is running and the surface is moving. Poor flow means poor oxygen.
Watch the tank for ten minutes. Is one fish chasing the surfer? Is the surfer facing its reflection?
Count your fish and measure the tank, and look up how big each species grows.

What to do next

Change one thing at a time and give each change three to five days, so you know what worked.

The common causes and fixes

Poor water: the commonest hidden cause. Keep up weekly water changes, don't overfeed, and make sure a new tank has finished cycling.
New tank or new fish: stress from moving usually fades in a few days. Acclimate new fish slowly (see acclimation) and keep lights low for the first day.
Reflections: territorial fish such as male bettas and cichlids may fight their own reflection. Add a background to the back and sides, turn on a room light so the tank isn't the brightest thing, or dim the tank light.
Tank too small: active fish like danios, barbs and rainbowfish need length to swim, and a short tank can make them pace. Rehome some fish or move to a longer tank.
Bullying: a fish being chased will pace looking for escape. Add plants, rocks and caves to break lines of sight, or separate the bully (see aggression).
Nowhere to hide: a bare tank is stressful for shy fish. Add plants, wood or floating plants.
Trying to escape: some fish explore the edges looking for a way out, which can end in jumping. Keep a tight lid (see jumping fish).
Hunger and begging: many fish learn that people mean food and dance at the front glass. If they're eating well and look healthy, that's normal. Underfed fish may forage restlessly; see feeding.
Wrong temperature: check the heater. Bettas, for example, want about 26–28°C (78–82°F).
Flow: some fish want more current, some much less. Match flow to the species.
Boredom: live or frozen foods that the fish has to hunt, and rearranged decor, give it something to do.

Cautions

Loaches and plecos often graze along the glass or swim up and down it in bursts, especially at night. If the water tests fine and they eat well, this is usually normal for them.
Shrimp swimming wildly around the tank are not glass surfing. That can signal poisoning or a water problem; see shrimp dying.
Don't medicate for glass surfing alone. Treat only if you find a disease.

Products that help

Liquid test kit: the first thing to reach for when behaviour changes.
Aquarium background film: cuts reflections on back and side panels.
Live or floating plants: shade, cover and broken sight lines.
Caves and driftwood: hiding places that cut stress and aggression.
Air stone or adjustable filter: for oxygen and the right flow.
Tight-fitting lid: stops escape artists jumping.

How to prevent it

Research adult size and swimming needs before you buy.
Cycle new tanks before adding fish, and add fish gradually.
Keep schooling fish in proper groups, usually six or more.
Give every tank plants or decor to hide behind.
Test water regularly, not just when something looks wrong.

Why it happened

Glass surfing is a fish's way of trying to get away from something: bad water, a bully, its own reflection, or a space that is too small. Sometimes it is just excitement about food. The behaviour itself does little harm, but the stress behind it can make fish sick (see stress).

When to get help

Ask an experienced keeper or a fish vet if surfing lasts more than a week after you've fixed water and setup, if several fish do it at once, or if it comes with other signs such as clamped fins, spots, weight loss or refusing food.

In the game

Multiple Tank Syndrome doesn't have a glass surfing card yet. The Rehome a fish card is the real-life answer when a tank is just too small or too crowded.