Troubleshooting
Start from the observation, not from a guess at the cause. Every page here lists several plausible explanations and — crucially — what separates them from one another.
Before anything else
Test ammonia, nitrite and nitrate, and check the temperature with a thermometer you trust rather than the heater dial. Water quality is behind a large majority of aquarium problems, and testing takes five minutes.
Water (5)
- Ammonia detected in the tankAmmonia above zero in a stocked tank means production has outrun the filter. Change water now, stop feeding, and then find out why — because the reading is the symptom, not the cause.Act now5 possible causes
- Nitrite detected in the tankNitrite means the first stage of the nitrogen cycle is working and the second is not keeping up. Change water, stop feeding, and add chloride only if the tank has no invertebrates or salt-sensitive fish.Act now3 possible causes
- Nitrate is highHigh nitrate means more nitrogen is entering than is leaving. It is rarely an emergency, but it is a reliable signal that maintenance, feeding or stocking needs adjusting — and it should be brought down gradually, not in one enormous change.Act within days5 possible causes
- Cloudy waterWhite or grey haze in a new tank is usually a bacterial bloom and clears itself. In an established tank it usually signals a change in the biological balance and is worth investigating rather than filtering out.Act today5 possible causes
- pH has crashedA pH crash means the carbonate buffer ran out. Test KH — if it is at or near zero, that is the whole explanation, and the fix is to restore buffer gradually rather than to add a pH-up product.Act now4 possible causes
Fish health (6)
- Fish gasping at the surfaceGasping means the fish cannot get enough oxygen into its blood. That can be because there is too little oxygen in the water, or because the gills or the blood have been damaged — most often by ammonia or nitrite. Test the water first.Act now5 possible causes
- White spots on a fishDiscrete salt-grain spots on the body and fins usually indicate whitespot. Several other conditions look similar, and treatment differs, so identify carefully before medicating.Act today4 possible causes
- Damaged or ragged finsRagged fins are a sign, not a diagnosis. Physical damage, nipping and bacterial fin erosion all look similar at first, and the treatment is different for each. Water quality is the common factor behind almost all of them.Act within days4 possible causes
- Fish is swimming abnormally'Swim bladder disease' is a description of a symptom, not a diagnosis. Buoyancy problems arise from digestion, from physical compression, from infection and from selective breeding for body shape, and the responses differ.Act within days4 possible causes
- Fish is bloatedBloating has several very different causes, and the one thing to check first is whether the scales are raised. Scales standing out from the body — 'pineconing' — indicates fluid accumulation and carries a poor prognosis.Act today4 possible causes
- Fish are dying and I do not know whyWork through the physical and chemical causes before the biological ones. Most unexplained deaths in established tanks trace to something that entered the water, a temperature failure, or a slow decline in water quality that was never tested for.Act now5 possible causes
Fish behaviour (4)
- Fish is not eatingRefusing food is one of the earliest and most general signs that something is wrong. Water quality, stress, the wrong food and illness all produce it, and the pattern — one fish or all of them, new or established — narrows it quickly.Act today5 possible causes
- Fish is hiding constantlyConstant hiding is usually a social or environmental problem rather than a disease. The commonest single cause is a shoaling species kept in too small a group.Act within days5 possible causes
- Fish is lethargicLethargy with clamped fins is a general distress sign. Test the water first — ammonia, nitrite and temperature account for most cases — then look for a specific illness.Act today4 possible causes
- One fish is attacking the othersMost aquarium aggression is a stocking problem rather than an individual's temperament. Group size, tank dimensions, sex ratio and breeding state explain the large majority of cases.Act within days5 possible causes
Invertebrates (3)
- Shrimp are dyingShrimp die from things that leave fish untouched: copper, unstable hardness during moulting, and rapid parameter changes. Check what has gone into the tank before assuming disease.Act now5 possible causes
- Snails are taking overA snail population is set by the food available, and the food available is set by how much you feed. A snail explosion is an overfeeding signal, and the population falls when the food does.Observe and monitor3 possible causes
- Snails are dyingShell erosion means the water is too soft or too acidic. Sudden death usually means copper or a parameter change. And an inactive snail is not necessarily a dead one — check before removing it.Act within days4 possible causes
Plants (4)
- Plants are meltingMost melting in new plants is conversion from emersed to submersed growth, not death. Leave the roots alone, remove the dissolving leaves, and wait for new growth.Observe and monitor4 possible causes
- Plant leaves turning yellowWhich leaves are affected narrows it fast. Old leaves yellowing usually means a mobile nutrient — nitrogen — is short; new leaves yellowing usually means an immobile one, most often iron.Act within days4 possible causes
- Holes in plant leavesPinholes in older leaves are the classic potassium deficiency sign. Ragged or chewed edges are something eating the plant, and the difference is visible up close.Act within days3 possible causes
- Plants are not growingPlant growth is limited by whichever input is shortest — light, carbon or nutrients. Adding more of the others will not help, and adding more light usually produces algae.Observe and monitor4 possible causes
Algae (2)
- Green waterGreen water is a bloom of free-floating single-celled algae. It is triggered by excess light plus a nutrient release, and it does not respond to water changes because the algae reproduce faster than you can dilute them.Act within days3 possible causes
- Algae is taking overAlgae grows where light exceeds what the plants can use with the carbon and nutrients available. The type of algae narrows the cause; reducing light and improving flow and maintenance fixes most cases.Act within days4 possible causes
Equipment (2)
- The filter flow has droppedReduced flow is nearly always mechanical — clogged media, a blocked intake, an airlock or a worn impeller. It matters biologically too, because a filter starved of flow loses oxygen and biological capacity.Act today4 possible causes
- Heater problem — tank too hot or too coldAquarium heaters fail in both directions and their dials are frequently inaccurate. A separate thermometer is the only reliable check, and a stuck-on heater can kill a tank within hours.Act now4 possible causes
Cycling (2)
- The cycle seems to have stalledA stalled cycle almost always has a specific chemical cause: pH too low, ammonia dosed too high, or something inhibiting the bacteria. It is rarely a matter of waiting longer.Act within days5 possible causes
- Fish dying in a new tankIn a tank under about three months old, the overwhelming probability is that it was not cycled. Test ammonia and nitrite before considering anything else.Act now4 possible causes