Storms, fireworks and power cuts: what actually reaches the water

Quick answer

Fish do not lie awake dreading thunder. They also do not ignore it. Sound travels far better in water than in air, and a fish detects a storm through its whole body long before you hear it through a window.

A fish holding station calmly, fins spread, using the whole tank. That is your baseline for comparison.

What actually harms fish during storms and firework nights is rarely the noise itself. It is oxygen, temperature, water chemistry and power. A still, muggy, low-pressure night is when pond fish suffocate. Cold rain into a warm pond causes a temperature crash. A power cut stops the filter, and the bacteria in it start dying within hours.

The noise contributes one specific danger: startled fish jump. Koi go over pond edges and into netting, and aquarium fish hit the lid or go out through a feeding hatch.

• Check oxygenation before a forecast storm, especially in warm weather. That is the risk that kills.
• Secure lids and pond netting. Jumping is the main injury mechanism on firework nights.
• Leave a dim light on rather than total darkness, so a flash is less of a change.
• Do not feed heavily before or during a storm, and do not feed at all during a power cut.
• Goldfish, koi, tetras, barbs and catfish hear far better than most other fish, for an anatomical reason.

Decide in 60 seconds

What a fish actually detects

A fish does not have an ear opening, and yet it lives in a medium that carries sound extraordinarily well. It picks up its acoustic world in two ways.

The lateral line. A row of sensory pits along each flank, and around the head, that detect water movement and low-frequency pressure changes. This is how a fish senses another fish approaching, a hand near the glass, or the pressure wave from a footstep on a floorboard. It is essentially a sense of touch extended a short distance into the surrounding water.

The inner ear. Fish have inner ear structures containing dense stones that move relative to the body when a sound wave passes, stimulating hair cells. In most fish this gives sensitivity to low frequencies at close range.

Here is where species matters. A large group of freshwater fish, including goldfish, koi, tetras, barbs, danios, rasboras, loaches and catfish, possess a chain of small bones linking the swim bladder to the inner ear. The swim bladder acts as a pressure receiver and the bones transmit its vibrations to the ear. Fish with this arrangement hear a considerably wider range of frequencies, and much more quietly, than fish without it.

The practical consequence is that a goldfish or a koi experiences a firework or a thunderclap far more vividly than a cichlid or a marine tang in the same conditions. If your fish are in that group, take the startle risk more seriously.

Sound also enters a tank through the stand and the floor, not only through the air. A subwoofer, a slammed door, or someone dropping something in the room above reaches the fish as a pressure event through the structure, which is why an aquarium in a room with a hard floor is louder for the fish than it seems to you.

What storms actually do

Oxygen falls.

This is the one that kills. Dissolved oxygen decreases as water warms, and warm fish need more of it because their metabolic rate rises with temperature. On a still, humid night there is no wind moving the surface, plants and algae are respiring rather than photosynthesising, and any decaying material in the pond is consuming oxygen as well. Low pressure ahead of a storm does not help. The result is a pond that reaches its lowest oxygen just before dawn, which is when fish are found gasping or dead.

Temperature changes abruptly.

Cold rain falling onto a warm pond cools the surface layer quickly. Fish that were in a stable environment at dusk are in a changed one by morning. In a heavily stratified pond, a sudden mixing of layers can also push oxygen-poor bottom water upwards.

Water chemistry shifts.

Rainwater is soft, low in minerals and slightly acidic. A large volume entering a small pond dilutes carbonate hardness, which is the buffer that keeps pH stable. Once the buffer is depleted, pH can move sharply, and rapid pH change is far harder on fish than a stable value at the wrong number.

Runoff arrives.

Water coming off a roof, a driveway, a patio or a lawn brings dirt, fertiliser, moss treatments, hydrocarbons and whatever else was on those surfaces. A pond fed by a downpipe is receiving all of it.

Debris blows in.

Leaves, blossom and garden material land in the water and begin decaying, which consumes oxygen and releases ammonia.

Power fails.

Filters, air pumps, UV units, heaters and protein skimmers all stop together.

Why a power cut is a biological problem

A filter is not just a pump. It is a colony of aerobic bacteria living on the media, converting ammonia to nitrite and nitrite to nitrate.

Those bacteria need oxygen, and in a running filter they get it from the water passing through. When the flow stops, the oxygen trapped in the media is consumed within hours. After that the colony begins to die and the media goes anaerobic, developing a foul smell.

That creates the trap that catches people after a long outage: restarting the pump flushes a slug of decomposed material and low-oxygen water into the tank all at once, which can be worse than the outage itself.

The plan during an outage.

Stop feeding. Fish need very little for days, and uneaten food plus a dead filter is the worst possible combination.

Keep the tank insulated. Blankets or towels around a tank hold heat surprisingly well. Do not open the lid unnecessarily.

Provide oxygen manually if you can. A battery-operated air pump is inexpensive and is the single best piece of emergency equipment for a fishkeeper. Failing that, gently pouring water from a jug from a height, or scooping and returning water repeatedly, moves the surface and adds oxygen.

If the outage runs long, take the filter media out and keep it submerged in tank water where you can aerate it, rather than leaving it sitting in a stagnant canister.

Restarting.

If the filter has been off for hours, smell the media before you restart. If it smells foul, rinse it in tank water, never in tap water, before returning it and starting the pump. Then treat the tank as if it were partly cycled again: test the water, feed lightly, and watch for ammonia.

Fireworks specifically

The noise reaches the water as a percussive pressure pulse, and the flashes reach it as sudden light in an otherwise dark environment. Both trigger the same reflex: a fast escape burst.

The damage comes from what the fish hits.

Jumping out. Koi and goldfish are strong jumpers and a startled fish clears a low pond edge easily. Indoors, fish go out through open lids and feeding hatches. Many fish deaths on firework nights are simply fish found on the floor in the morning.

Hitting hard surfaces. Rocks, filter intakes, decor and the tank wall. The typical injury is a scraped patch on the flank or head, a damaged eye, or torn fins, and the real problem is the secondary bacterial or fungal infection that follows a break in the slime coat.

Getting tangled. Pond netting that is loose or in contact with the water surface catches fish and can drown them. Netting should be taut and held clear of the surface.

Fallout. Fireworks deposit paper, ash and chemical residue over a wide area. A pond directly downwind of a large display collects it, and a partial cover on the night removes most of it.

Dense planting, shaded areas and low hides give a startled fish somewhere to go that is not the wall or the lid.

What to do before a known noisy night

Aerate more, and start early. An extra air stone, a spray bar angled at the surface, or a pond pump raised to break the surface. Surface agitation is what actually exchanges gas, so a rippled surface matters more than bubbles in the water column.

Secure every opening. Full lid, feeding hatch closed, and any gap around cables covered. On a pond, taut netting well clear of the water, and a check of the lowest section of the edge.

Reduce the light contrast. The most violent startle happens when a flash hits a fish in complete darkness. Leaving a dim room light or a low outdoor light on reduces the difference between the flash and the background. Do not leave the tank light itself blazing all night; that disrupts the day and night cycle and stresses the fish in a different way.

Do not feed a big meal. Digestion consumes oxygen, uneaten food fouls the water, and a stressed fish is unlikely to eat properly anyway. Skip or reduce the evening feed.

Add cover. Dense planting, floating plants, a length of pipe, or a shaded overhang gives fish somewhere to go that is not the glass or the surface. A fish with a hide startles less violently and stops sooner.

Decouple the tank from the floor. A dense foam mat under the stand reduces how much structural vibration reaches the water.

Check the pond edges and the outflow. A blocked overflow during heavy rain is how ponds flood and fish end up in the garden.

What a stressed fish looks like, and how long it should last

During and immediately after. Dashing, sudden bursts of speed, crashing into decor, hiding, freezing on the bottom, clamped fins held tight against the body, and colour fading. In pond fish, jumping.

Next day. Reduced appetite, remaining in cover, reduced activity. This is normal and should improve.

Two or three days on. A fish still hiding, still refusing food, still clamped, or breathing rapidly is no longer showing a startle response. Test the water, because the more likely explanation is that something changed chemically during the same weather.

Days to weeks on. White cottony growth, red streaks in the fins, cloudy patches on the skin, or a fuzzy edge to a damaged fin. These are secondary infections at the site of an injury, and they are the most common delayed consequence of a noisy night.

Fins carried open and away from the body, steady gill movement, normal colour. Clamped fins and faded colour are the signs to act on.

What differs by setup

Ponds are the most exposed: rain, runoff, debris, temperature swings, predators arriving while fish are distracted, and no lid. They are also the setting where oxygen crashes actually kill.

Goldfish and koi hear well, jump powerfully, and live in the environment most affected by weather. Treat them as the highest-risk group.

Small aquariums change temperature and chemistry faster than large ones because there is less water to buffer the change. A small tank in a room that loses heat during a power cut is in trouble sooner.

Heavily planted tanks already run an oxygen cycle that dips at night. Adding a warm, still night on top of that narrows the margin further, so aeration overnight is worth more in a planted tank than in a bare one.

Marine tanks depend on the protein skimmer and the return pump for gas exchange as well as filtration, and reef inhabitants tolerate change poorly. A battery air pump and a plan for the outage matter more here, not less.

Labyrinth fish, such as bettas and gouramis, can take air from the surface and are less vulnerable to an oxygen crash, provided the surface is accessible. Do not seal the lid so completely that they cannot reach it.

What never to do

Do not tap the glass, ever, and least of all to check on a startled fish.

Do not do a large water change in the middle of a storm or immediately after a power cut without testing first. Stacking a chemistry change on top of an oxygen or temperature event is how a survivable night becomes a fatal one.

Do not restart a filter that has been off for many hours without checking and rinsing the media in tank water.

Do not top a pond up with a hose during heavy rain without a dechlorinator, and do not deliberately fill a pond with collected rainwater without knowing what it does to your hardness and pH.

Do not feed during a power cut.

Do not seal a tank completely airtight in an attempt to keep noise out. Gas exchange at the surface is what keeps the fish alive.

Do not add stress-coat products, salt or medication as a precaution. Nothing in a bottle prevents a startle response, and adding chemistry to a tank that is already changing makes the situation less predictable.

When to get help

Act immediately yourself, rather than waiting for advice, if fish are gathered at the surface gasping. Maximum aeration and a partial water change with temperature-matched, dechlorinated water is the response, and it is time-critical.

Contact an aquatic vet or an experienced fishkeeping specialist if fish have visible wounds, cloudy or red patches, cotton-like growth, or damaged eyes in the days after an incident. Injuries that break the slime coat become infections and those need treatment rather than time.

Contact one urgently if several fish die in a short period, if survivors are lying on the bottom breathing rapidly, or if water tests show ammonia or nitrite after a power cut, since that means the filter has not recovered and the fish are being poisoned by the water they are in.

The goal is a fish that has somewhere to go, water that holds enough oxygen, and a lid it cannot get past.