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Problems

When something goes wrong in the tank

Plain-words guides to keeping fish well, and to fixing things when they aren’t: how to spot it, what to do right now, and how to stop it coming back.

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Why are my fish dying?

A calm checklist for when fish are dying and you don’t know why.

Care basics

Get these right and most problems never start. They’re the same things you juggle in the game: heat, pH, flow, space and food.

Cycling a new tankA new tank needs weeks to grow the bacteria that make fish waste safe. How to cycle without fish, save fish in an uncycled tank, and know it's done. Heating and coolingMost tropical fish want about 24–26°C. How to choose and size a heater, where to put it, how to cool a tank in summer, and how to match fish by temperature. Changing pH safelyNeed to lower or raise aquarium pH? Stability matters more than an exact number. Test KH first, then change it slowly with RO water or crushed coral. Fish size and tank sizeThe fish in the shop is a baby. Check its adult size first, then pick a tank long and wide enough for that adult. Why "1 inch per gallon" is only a rough guide. Bottom, middle and top fishFish live at the top, in the middle or on the bottom of a tank. Stock all three levels without crowding any one, and feed each level where it eats. Plants and spaceA good layout gives fish places to hide, open water to swim in and broken sight lines that calm fights, while leaving room to clean and for water to flow. Water flow and currentsSome fish need a strong current, others want calm water. Learn to read your tank's flow, fix dead spots, and add or tame current safely. What fish eatFish can be meat eaters, plant eaters or both, and feed at the top, middle or bottom. Here's how to pick foods that match each fish, and keep them fresh. How often and how much to feedMost adult fish need one small meal a day, only what they finish in about two minutes. Here's how to make sure every fish, shy or bottom-dwelling, eats.

Sicknesses and problems in the game

Every one of these is a card in Multiple Tank Syndrome, and a real thing that kills real fish.

disease Ich (white spot)White grains of salt on your fish? That's ich. How to confirm it, treat the whole tank, and why you keep treating after the spots are gone. Act today disease Velvet (gold dust disease)A fine gold or rusty dust on your fish, with fast breathing? That may be velvet. How to check with a flashlight, and how to treat it before it kills. Act within the hour disease Fin rotFrayed, shrinking or white-edged fins are usually fin rot. Fix the water first, then treat. How to tell mild from serious, and when fins grow back. Act today disease Columnaris (cotton wool mouth)White fuzz on the mouth or a pale saddle across the back can be columnaris, a fast bacterial disease. How to tell it from fungus and what to do today. Act within the hour disease Hole in the head (HITH)Pits and holes on your fish's head or along its side? That's hole-in-the-head. What causes it, how to treat it, and what we know about carbon. Act this week disease Gill flukesFast breathing, flashing and one gill held shut can mean gill flukes. How they spread, how praziquantel works, and why one dose isn't enough. Act today disease Camallanus wormsRed threads sticking out of your fish's vent are camallanus worms. How they spread, why you treat the whole tank, and how to deworm properly. Act today water Ammonia spikeAmmonia reading above zero and fish gasping? What to do in the next hour, why pH and temperature make ammonia deadlier, and how to stop it coming back. Act within the hour water Chlorine and chloramineAdded tap water without conditioner? Dose dechlorinator for the whole tank right now. Why chloramine doesn't evaporate, and how to protect fish next time. Act within the hour accident Soap and contaminantsSoapy hands, hand cream, bug spray or cleaner got into your tank? What to do in the next hour, how carbon helps, and how to keep it from happening again. Act within the hour disease Dropsy (pinecone scales)A swollen fish with scales sticking out like a pinecone has dropsy. What causes it, what you can try, the honest outlook, and how to euthanise humanely. Act today water Algae takeoverGreen water, brown dust, hair, black tufts or blue-green slime? Identify your algae, fix the cause (light, nutrients, flow) and clean up without hurting fish. Act this week water pH crash (old tank syndrome)pH has dropped below 6.5 in a tank that hasn't had water changes in months? Why KH runs out, and why fixing it too fast kills fish. A safe step-by-step plan. Act today equipment Cold tank / heater failureTank colder than it should be? How to warm it safely (slowly), why a cold snap often leads to ich, and how to size heaters so it doesn't happen again. Act within the hour disaster Power failurePower out and no filter, heat or air? Oxygen first, then warmth. Don't feed, protect the filter bacteria, and what to do when the power comes back. Act within the hour

More problems, not in the game

Diseases, water trouble, algae, behaviour and equipment, plus how-tos for everyday jobs.

Diseases and parasites 18

Anchor wormsThin threads sticking out of a fish, with a red sore at the base? Anchor worms. Pull them carefully, then treat the whole tank or pond for weeks. Act today Betta fin rotRagged, shrinking betta fins? Clean warm water fixes most fin rot. How to tell rot from fin biting, when to medicate, and what healing looks like. Act today Black spot diseasePepper-like black dots on pond or wild fish are usually fluke larvae (black grub). They can't spread in a home tank and are mostly cosmetic. Not urgent Bloated or constipated fishSwollen belly? Look from above: flat scales usually means constipation or eggs; pinecone scales means dropsy. Fasting, daphnia and when to worry. Act today Costia (Ichthyobodo)Costia is a tiny skin and gill parasite that causes a blue-grey slime coat and flashing, and can kill young fish fast. How to confirm it and treat it. Act today Fish liceFlat, see-through discs crawling on your fish? Those are fish lice. Remove them, treat the whole tank or pond for weeks, and scrub away their eggs. Act today Fish TBFish TB (mycobacteriosis) causes wasting, bent spines and sores, and has no reliable cure. It can infect people through cuts. Protect fish and yourself. Act this week Fish fungus (white fuzz)White cotton-like fuzz on a fish is usually a water mould growing on a wound, but it can be columnaris (bacterial). How to tell and treat each. Act today Hexamita (Spironucleus)Hexamita are gut parasites behind white stringy poop and thin cichlids. Why medicated food beats water dosing, the hole-in-the-head link, and vet help. Act today Internal wormsA fish that eats well but stays thin, or has white stringy poop, may have internal worms. How to choose the right dewormer and keep shrimp and snails safe. Act this week LymphocystisLymphocystis is a virus causing white, cauliflower-like lumps on fins and skin. It usually clears in weeks. Never cut it off; fix stress and water. Act this week Malawi bloatA swollen belly, no appetite and white stringy poop in mbuna or Tropheus. Act fast: isolate the fish, call a vet about metronidazole, and fix the diet. Act today Neon tetra diseaseNeon tetra disease is a parasite with no reliable cure. Learn the signs, how to tell it from bacterial false neon disease, and how to protect your tank. Act today Popeye and cloudy eyeFish eye bulging or cloudy? One eye usually means injury; both eyes usually mean water problems or infection. Fix the water first, then treat. Act today Septicemia (red streaks)Red streaks in fins and red blotches on the body can mean a blood infection. It's serious: isolate the fish, fix the water and call a vet today. Act today Swim bladder problemsFish floating, sinking or swimming sideways? Swim bladder disease is a symptom, not one disease. Test the water, fast for 2–3 days, then feed less. Act today Trichodina and ChilodonellaTrichodina and Chilodonella are skin and gill parasites that cause slime, flashing and gasping. How to confirm them, treat them and fix the cause. Act today Lumps and tumoursA lump on your fish could be a tumour, cyst, virus, parasite or abscess. How to tell them apart, what a vet can do, and how to judge quality of life. Act this week

Water problems 14

Cloudy waterMilky, green or tea-coloured water? The colour tells you the cause. Most cloudiness is harmless, but cloudy plus a bad smell or gasping fish is an emergency. Act today OverfeedingCloudy water and gasping fish after a big feed? How overfeeding crashes a tank, how much to really feed, and how to handle gel foods like Repashy. Act today Bubbles, foam and surface filmMost bubbles are normal: filters, pearling plants, betta nests. Foam that won't pop or an oily film means waste, overfeeding or soap. How to fix it. Act this week Copper in the tankCopper in a fish tank can kill shrimp and snails at tiny levels. Find the source (pipes, medicines, fertilizer, coins), test for it, and remove it safely. Act today White fuzz on driftwoodWhite fuzz or slime on new driftwood is usually harmless biofilm that fades in a few weeks. How to remove it, what eats it, and signs the wood is rotting. Not urgent Gas bubble diseaseTiny bubbles in fins, under the skin or behind the eyes mean the water holds too much gas. Find the air leak or cold-water cause and degas the water today. Act within the hour Fish gasping for airFish gasping at the surface is an emergency. Add air, check the temperature, test for ammonia and nitrite, and change water with conditioner now. Act within the hour Goldfish changing colourYoung goldfish often change colour naturally. Sudden black patches can be healing ammonia burns, and fading can mean too little light. Test the water first. Act today High nitratesKeep nitrate under about 20–40 ppm for most fish, lower for shrimp and sensitive species. Test your tap water, change water, feed less and add plants. Act this week Nitrite poisoningNitrite above zero in a new tank stops fish blood carrying oxygen. Change water daily, add a measured dose of salt, and let the tank finish cycling. Act within the hour Shrimp dyingCherry shrimp dying one by one or all at once? Check for failed molts, copper, flea treatments and sprays, big water changes and unstable GH. What to do today. Act today Fish tank smells badA healthy tank smells earthy. Rotten eggs means hydrogen sulphide; a sharp rotten smell means something died. Find the source and fix it safely. Act today Yellow or brown water (tannins)Tea-coloured water from driftwood or leaves is tannins: harmless and loved by blackwater fish. How to tell, and how to clear it if you don't like the look. Not urgent Distilled, RO, rain or well water?Can you use distilled, RO, rain or well water in a fish tank? Why pure water needs minerals added back, how to mix it with tap water, and well-water risks. Not urgent

Algae and pests 10

Black beard algaeDark, tough tufts on leaves, wood and the filter? That's black beard algae. Spot-treat it, trim it, and steady your CO2 and flow so it doesn't return. Not urgent Brown algae (diatoms)Brown dust on glass, sand and leaves in a new tank? It's diatoms. They're harmless, wipe off easily, and usually fade on their own within weeks. Not urgent Cyanobacteria (blue-green slime)Slimy blue-green or dark red sheets that smell bad? That's cyanobacteria, not algae. Siphon it out, black out the tank, and fix flow and nutrients. Act this week DuckweedDuckweed soaks up nitrate and shades fish, but it spreads fast and can cover the surface. How to keep it in check, remove it, and never let it escape. Not urgent Green spot and green dust algaeHard green dots on glass and slow plants are green spot algae; check phosphate and scrape. A soft green film is green dust algae: wait, then wipe. Not urgent Hair algaeHair algae means light and nutrients are out of balance. Pull it out by hand, cut the light, check iron and phosphate, and add Amano shrimp. Act this week HydraTiny tentacled creatures stuck to the glass? Hydra sting and eat fry and baby shrimp. Cut feeding first, then treat if needed, protecting your snails. Act this week Planaria and white wormsWhite worms on the glass? Most are harmless detritus worms. Planaria glide and have an arrow-shaped head; they're a threat to shrimp eggs and babies. Act this week Pest snailsTiny snails everywhere? A snail boom almost always means too much food. Feed less, trap and remove them, and avoid copper snail killers. Not urgent Staghorn algaeStaghorn algae is grey-green, branching red algae. It points to unstable CO2, weak flow or built-up waste. Remove it, spot-treat, then fix the cause. Act this week

How-to guides 12

Cleaning your tankA safe cleaning routine: change 20–30% of the water weekly, vacuum the gravel, match temperature, use dechlorinator, rinse filters in tank water. Not urgent Acclimating new fishBringing new fish home? Float the bag to match temperature, don't pour bag water in, keep lights off, and quarantine first. Shipped fish need special care. Act today Aquarium saltAquarium salt is plain sodium chloride. It helps with nitrite, some parasites and hospital tanks, but is no tonic. Doses, cautions and sensitive fish. Not urgent Found a dead fishFound a dead fish in your tank? Remove it now, test for ammonia, check the others, and dispose of it in the bin or by burial. Never flush it. Here's how. Act within the hour Disinfecting a tankStep-by-step bleach and alcohol methods for disinfecting an empty fish tank, nets and equipment, what to throw away, and when leaving a tank empty is enough. Act this week Hybrid fishHybrid fish can't replace wild species. How hybrids happen in home tanks, why pure lines of fish like the zebra pleco matter, and what to do with hybrid fry. Not urgent How long fish liveMost small aquarium fish live 2–5 years, bettas 3–5, and goldfish 10–15 or more. A lifespan table, signs of old age, and how to keep an old fish comfortable. Not urgent Moving a fish tankMoving a fish tank across the room or to a new house: bag the fish, keep the filter media wet, move the tank empty and set it up again the same day. Not urgent Cats, dogs and kidsCat in the fish tank? A secure, weighted lid fixes most pet and child problems. Plus how to stop overfeeding and keep flea treatments and sprays away. Act today Quarantine tankA hospital or quarantine tank keeps new and sick fish apart so disease can't reach your main tank. How to set one up, how long to quarantine, and how to dose. Not urgent Are my rocks safe?How to tell if a rock is safe for your fish tank: the vinegar fizz test, metal veins, sharp edges, collecting rocks outdoors, and safe vs risky types. Not urgent Going on vacationHealthy adult fish can skip a few days of food. For longer trips, use a tested auto feeder or a friend with pre-portioned food, not a feeder block. Not urgent