Where to start
How to treat ich in fish: white spot disease, step by step
The most common fish parasite: how to confirm white spot, raise the heat safely and finish the treatment.
Diseases & algaeFin rot treatment: how to stop it and regrow healthy fins
Ragged fins in bettas and community fish: stages, fin rot vs fin nipping and when to medicate.
Diseases & algaeAmmonia poisoning in fish: signs, emergency treatment and prevention
The emergency guide for gasping fish and new tanks with ammonia or nitrite readings.
Diseases & algaeSwim bladder disease in fish: symptoms, causes and treatment
Floating, sinking or upside-down fish, with feeding fixes for goldfish and bettas.
Diseases & algaeBlack beard algae: how to identify, remove and prevent BBA
Why this tough red algae appears and how to remove it without harming plants.
Diseases & algaeFish tank cloudy water: why it happens and how to fix it
White, green or brown cloudy water: what each color means and how to clear it.
01
Common fish diseases: start with the water
Sick fish are usually a symptom, not the root problem. Ammonia, nitrite, a temperature drop or a recent move weaken the immune system, and parasites or bacteria that were already present take over. That is why every guide in this section starts with a water test and why a reliable liquid kit from our aquarium test kit reviews is the first tool to own.
If several fish are gasping, hanging at the surface or dying in a new tank, go straight to ammonia poisoning in fish and our guide to new tank syndrome. A water change with a good water conditioner is the safest first step in almost every emergency.
02
Identify the problem by its symptoms
Use this table to narrow things down, then open the matching guide for the full treatment.
| Main symptom | Likely problem | Guide |
|---|---|---|
| White salt-like dots, scratching | Ich (white spot) | Ich treatment |
| Ragged, shrinking fins | Fin rot | Fin rot treatment |
| Floating, sinking or swimming on the side | Swim bladder problem | Swim bladder disease |
| Gasping, red gills, sudden deaths | Ammonia or nitrite | Ammonia poisoning |
| Pale patches and restless swimming in tetras | Neon tetra disease | Neon tetra disease |
| Rapid breathing, flared gills, scratching gills | Gill flukes | Gill flukes |
03
Algae problems
Algae is not a disease, but it tells you something about light, nutrients and maintenance. Brown dust on a new tank is normal and usually fades, see brown algae in a fish tank. Pea-soup water points to excess light and nutrients, covered in green water in a fish tank. Stubborn dark tufts on wood and leaf edges are black beard algae, often linked to unstable CO2. Algae-eating helpers from our list of the best snails for algae help, but they never replace fixing the cause.
04
Pests and water problems
Small white worms on the glass are usually harmless detritus worms and a sign of overfeeding, while flat worms with a triangular head are planaria, a real risk in shrimp tanks. Thread-like parasites hanging from a fish’s skin are anchor worms and need targeted treatment.
For milky or hazy water, read cloudy fish tank water: the cause is usually a bacterial bloom in a new tank, overfeeding or disturbed substrate, and the fix is patience plus less food rather than chemicals.
05
Medication basics and when to see a vet
Treat only once you are reasonably sure what you are treating, use one medication at a time, remove carbon from the filter and always follow the label for dose and duration. Many medications harm shrimp and snails, so check before dosing. A quarantine tank for new fish prevents most outbreaks in the first place.
For serious or fast-moving cases (fish dying daily, body ulcers, swelling with raised scales, or no improvement after a full course), contact an aquatic veterinarian. A vet can examine skin and gill samples under a microscope and prescribe drugs that are not sold over the counter.
Every guide in this section
- 01How to treat ich in fish: white spot disease, step by step
- 02Fin rot treatment: how to stop it and regrow healthy fins
- 03Swim bladder disease in fish: symptoms, causes and treatment
- 04Ammonia poisoning in fish: signs, emergency treatment and prevention
- 05Neon tetra disease (NTD): symptoms, look-alikes, treatment and prevention
- 06Dwarf gourami disease (DGIV): symptoms, causes and what you can do
- 07Black beard algae: how to identify, remove and prevent BBA
- 08Brown algae in fish tank: why diatoms appear and how to get rid of them
- 09Green water in fish tank: causes, fast fixes and prevention
- 10Fish tank cloudy water: why it happens and how to fix it
- 11Detritus worms in your aquarium: what they are and how to get rid of them
- 12Planaria worms in your aquarium: how to identify and get rid of them
- 13Anchor worm in fish: how to spot and treat Lernaea
- 14Gill flukes in fish: symptoms, diagnosis and treatment
Frequently asked questions
Frequently asked questions
What are the most common freshwater fish diseases?
The most common freshwater fish diseases are ich (white spot), fin rot, swim bladder problems, fungal infections and columnaris, plus ammonia poisoning in new or overstocked tanks. Most of them are triggered or made worse by poor water quality and stress, so testing the water and changing it is always the first step before choosing a treatment.
How do I know if my fish is sick?
Watch for changes from normal behavior: hiding, loss of appetite, clamped fins, scratching against objects, gasping at the surface or swimming oddly. Physical signs include white spots, ragged fins, cloudy eyes, red streaks, swelling or raised scales. One sign alone may be stress; several together usually mean illness and a water test.
Should I treat the whole tank or just the sick fish?
It depends on the disease. Parasites such as ich are already in the water, so the whole tank needs treating. Bacterial infections like severe fin rot are better treated in a hospital tank, especially with antibiotics that can harm the filter bacteria. Always remove shrimp and snails if the medication is not labeled safe for them.
Can I use aquarium salt for every fish disease?
No. Salt helps with some parasites and stress in fish that tolerate it, such as livebearers and goldfish, but it harms scaleless fish, many catfish, loaches and live plants, and does little against internal or serious bacterial infections. Use it only for a specific reason, at a dose given by the product label.
Written and reviewed by the handsoffmyhobby editorial team according to our editorial policy.
