Aquarium tools · ProblemsUpdated October 10, 2026

Aquarium Problem Checker

Pick the symptoms you see and, if you can, enter your water test results. The checker sorts problems by urgency, lists the most likely causes, and gives safe first steps based on veterinary and extension guidance.

What you described

Live preview

Pick symptoms and the tank will show them.

What are you seeing?

Fish

Water

Water test results (optional, but they help most)

Ammonia

mg/L

Nitrite

mg/L

Nitrate

mg/L

Likely causes and first steps

Select one or more symptoms above.

This guide points to common causes and safe first steps. It cannot diagnose disease; for sick or dying fish, contact a vet with fish experience or an aquatic specialist.

Quick answer

Start by testing the water: most aquarium problems trace back to ammonia, nitrite, low oxygen or temperature. Gasping fish, sudden deaths and bloating are urgent, and so is white spot, which spreads fast (UF/IFAS). Cloudy, green or tea-coloured water is usually not dangerous on its own. Any reading of ammonia or nitrite above zero calls for a partial water change.

Common aquarium problems at a glance

Symptom, urgency and first check
Symptom	Urgency	Check first
Fish gasping at the surface	Act now	Oxygen, nitrite, temperature
Sudden deaths	Act now	Ammonia, nitrite, temperature, recent changes
Bloated or swollen fish	Act now	Isolate the fish; get vet advice
White spots like salt grains	Act now	White spot (ich): treat the whole tank
Fins ragged or rotting	Act today	Water quality and fin-nipping tankmates
Flashing (rubbing on objects)	Act today	Ammonia, nitrite and parasites
Lethargic fish	Act today	Temperature and water quality
Cloudy white water	Not an emergency	New tank or overfeeding
Green water	Not an emergency	Light hours and sunlight
Tea-coloured water	Not an emergency	Driftwood tannins

Which water test results are a problem?

Freshwater test results and what to do
Test	Target	Act when
Ammonia	0 mg/L	Anything above 0 in an established tank
Nitrite	0 mg/L	Anything above 0
Nitrate	As low as practical	Above 50 mg/L
Dissolved oxygen	Above 5 mg/L	Below 5 mg/L (Merck)

Merck notes nitrite stops blood carrying oxygen, so gasping can be caused by nitrite as well as low oxygen.

White spot (ich): why the full course matters

White spot disease is caused by the parasite Ichthyophthirius multifiliis. UF/IFAS describes outbreaks as a true emergency because the parasite multiplies quickly in a closed tank. A microscope confirms it, but the salt-grain spots on fins and body are typical.

Only the free-swimming stage can be killed by treatment; parasites under the skin and in cysts are protected. That is why medicines are repeated over several days and why you should keep treating until the full course is done, even after the spots fall off. Quarantining new fish is the best prevention.

Safe first steps for almost any problem

Partial water change (25–50%) with dechlorinated water at the same temperature.
More surface movement to raise oxygen.
Feed less for a few days; remove uneaten food.
Do not clean all the filter media at once; rinse it gently in old tank water if needed.
Change one thing at a time so you can see what helps.

Frequently asked questions

Why is my fish tank water cloudy?

White or grey cloudiness is usually a bacterial bloom in a new or overfed tank, or dust from unrinsed gravel. Test ammonia and nitrite, feed less, and leave the filter alone; blooms usually clear within days to a couple of weeks.

Why are my fish gasping at the top of the tank?

Low oxygen is the most common cause, especially in warm, crowded tanks with a still surface. Nitrite poisoning and gill disease cause the same signs (Merck). Increase surface movement straight away and test nitrite.

How do I get rid of green water?

Tropica suggests 2–3 days of darkness and about 30% water changes every second day. Cut lighting hours and keep the tank out of sunlight; a UV sterilizer clears green water quickly.

When should I take my fish to a vet?

When fish are dying, have growths, ulcers or swelling, or do not improve after you fix water quality. Many vets now treat fish; an aquatic specialist can also help.

Can I use salt for sick fish?

Some freshwater treatments use aquarium salt, but many plants, catfish and invertebrates are sensitive to it. Use the salt calculator for exact doses and follow product directions.

What to calculate next

Sources

Disclaimer: This checker suggests likely causes and safe first steps; it cannot diagnose disease. For sick or dying fish, consult a veterinarian with fish experience or an aquatic specialist. See our disclaimer.

More aquarium tools

All aquarium tools