Knowledge Centre · Fish health and behaviour
Why is my fish lying on the bottom?
A fish resting motionless on the bottom is sometimes completely normal — many species rest there routinely, and most fish are less active overnight or in a dim tank — but it can also be an early sign of poor water quality, stress or illness, particularly if it's unusual for that fish or comes with other changes. This guide explains how to tell the difference and what to check first. It isn't a diagnosis for your specific fish.
At a glance
What it usually means
Often normal rest, especially for bottom-dwelling species or at night — but can signal water-quality stress, illness or temperature shock if it's out of character.
Test first
Ammonia, nitrite and temperature, the same day, if the behaviour is unusual for that fish or combined with other signs.
What this page won't do
Tell you which specific cause applies to your fish, or replace watching for further signs like laboured breathing or clamped fins.
Understanding this behaviour
Not every fish resting on the substrate is showing a problem. Bottom-dwelling species — corydoras, loaches, many gobies and similar fish — spend a large part of their normal day resting on or near the bottom, and this is simply how they live, not a sign of distress. Most fish, including ones that swim actively during the day, also become noticeably calmer or settle low in the water overnight or when the room is dark, which is a normal rest period rather than lethargy.
What's worth paying attention to is a change from a fish's own normal pattern: an active mid- or top-swimming fish suddenly spending most of daylight hours motionless on the bottom, a fish that doesn't respond normally when you approach the tank or add food, or resting that looks different from settled — tipped to one side, fins clamped tight, or unresponsive to a light tap on the glass. Context and the individual fish's usual behaviour matter more than the position alone.
Likely causes
Normal behaviour:
- Species-typical resting. Many bottom-dwelling and nocturnal species rest on the substrate for large parts of the day as standard behaviour.
- Overnight or low-light rest. Most fish are calmer and less active in the dark, similar to sleep, and may settle low in the water column as a result.
Water-quality related:
- Elevated ammonia or nitrite. Both are stressful and can leave a fish visibly lethargic well before more dramatic signs appear. See our ammonia and nitrite guides.
- Temperature shock or an unsuitable temperature. A sudden change, or a sustained temperature outside a species' normal range, commonly reduces activity.
- Low dissolved oxygen. Reduced oxygen availability affects general activity levels, not just breathing rate — see our breathing heavily guide if gill movement also looks abnormal.
Other possible causes:
- Stress from bullying, overcrowding or recent transport. A fish that's being outcompeted or harassed by tankmates, or recently acclimatised, may spend more time resting low and out of the way.
- General illness. Lethargy is a common, non-specific early sign across many fish illnesses, sometimes appearing before a more identifiable symptom does.
- Buoyancy or balance problems. If the fish can't hold its normal position or keeps tipping, see our swim bladder guide rather than treating it as simple resting.
Immediate actions
Establish whether it's a change from normal
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Check the species and the time of day
If this is a bottom-dwelling species, or it's overnight or in a dim room, this may simply be normal rest rather than something to act on immediately.
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Observe from a distance without disturbing the fish
Watch for a minute or two before tapping the glass or adding food. Note whether it's upright, breathing normally, and reacting to movement nearby.
If it looks unusual, test and check
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Test ammonia, nitrite and temperature
These are quick to test and among the most common underlying causes of unusual lethargy.
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Check for other signs
Clamped fins, rapid or laboured breathing, not eating, or visible marks all add useful context — see our Fish Symptoms hub if more than one sign is present.
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Check for bullying or overcrowding
Watch whether tankmates are chasing or outcompeting the affected fish, particularly at feeding time.
What to check next
If the cause still isn't clear after the steps above, work through this checklist:
- Is it one fish, or several fish resting low at the same time? Multiple fish affected points more toward a shared environmental cause; one fish points more toward an individual cause.
- Has anything changed recently — a new fish added, a water change, a filter or equipment issue, or a change in feeding?
- Does the fish respond normally when approached, or is it unresponsive?
- Is it resting upright in its normal position, or tipped, on its side, or struggling to hold position?
When it may be more serious
Common mistakes
- Assuming a bottom-dwelling species resting on the substrate is unwell, without checking whether that's normal for its type.
- Assuming an unusual case is "just resting" without testing water quality first.
- Adding medication before testing ammonia, nitrite and temperature.
- Repeatedly disturbing the fish to check on it, which adds stress on top of whatever's already affecting it.
- Overlooking bullying from tankmates as a cause because no single fish looks obviously aggressive at any one moment.
Prevention
- Learn each fish's normal resting behaviour and preferred position, so a genuine change is easier to notice early.
- Test ammonia and nitrite on a regular schedule, rather than only reaching for a kit once something already looks wrong.
- Avoid sudden temperature swings, and match temperature when doing water changes or acclimatising new fish.
- Stock with tankmates of compatible temperament and size to reduce the risk of ongoing bullying.
- Quarantine new arrivals before adding them to an established tank.
Frequently asked questions
Is it normal for fish to rest on the bottom of the tank?
Often, yes. Bottom-dwelling species such as corydoras, loaches and many gobies spend much of their day resting on the substrate as normal behaviour, and most fish rest more, sometimes near the bottom, overnight or in a darkened tank. What's worth investigating is a fish that doesn't normally do this suddenly staying there, especially in daylight, or resting in a way that looks abnormal rather than settled.
What's the difference between a fish resting and a fish lying on its side?
A resting fish is usually still upright, in its normal swimming position, and reacts normally to a hand near the glass or food being added. A fish lying on its side, or tipped at an odd angle, has lost normal control of its position in the water and is a more concerning sign, regardless of species or time of day.
How is this different from a fish hiding?
Hiding describes a fish retreating to cover, such as behind decor or plants, generally out of sight. Lying on the bottom describes a fish resting motionless in open water or on open substrate, still visible, rather than seeking shelter. The two can overlap, and either can be normal or concerning depending on context, but they're different behaviours worth telling apart.
How is this different from swim bladder disorder?
Swim bladder disorder specifically affects buoyancy control, so an affected fish often struggles to stay upright, floats when it shouldn't, sinks awkwardly, or swims at an odd angle. A fish simply resting upright on the bottom without any of those buoyancy problems isn't showing signs of swim bladder disorder. See our swim bladder guide if buoyancy control looks affected.
Should I test my water immediately if I see this?
If it's a species or time of day where resting on the bottom is normal, watch first rather than reacting immediately. If it's unusual for that fish, affects more than one fish, or comes with other signs such as clamped fins, rapid breathing or not eating, test ammonia, nitrite and temperature the same day rather than waiting to see if it resolves on its own.
How AquaPulse Water Check can help
A fish lying on the bottom could mean several different things, and the fastest way to narrow it down isn't a better guess — it's better context: how does today's ammonia, nitrite and temperature compare with what this tank normally reads? Test with your own kit and thermometer, log it in AquaPulse, and you get an answer that's specific to your tank's own history rather than a generic safe-range number that may not reflect what's actually normal here.
Water Check works from the readings you enter — it can't observe your fish's behaviour directly, and isn't a substitute for testing your water with a reliable kit, or for professional or veterinary advice when livestock are seriously or persistently unwell. Get AquaPulse on the App Store or Google Play to try it, or see the Water Check feature page for more detail.
Sources and editorial information
- Published
- 31 July 2026
- Last reviewed
- 31 July 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This guide is written and maintained by the AquaPulse team at Valeon Labs Ltd. It draws on published veterinary references rather than a single author's personal experience, and we paraphrase and link to those sources rather than reproducing their wording. We'll update this page if the sources it's based on change, or if we find something we've got wrong.
Good to know: This guide provides general informational guidance only. It is not a veterinary service and does not replace specialist or aquatic health advice for serious livestock health issues.
- [1] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for ammonia, nitrite, temperature and dissolved-oxygen stress as common causes of reduced activity and lethargy.
- [2] Merck Veterinary Manual — "Parasitic Diseases of Fish" Used for lethargy as a common, non-specific early sign across fish illnesses generally.