Aquarium Fish Diseases: Symptoms, Causes and What to Do

Spotting a fish with white specks, ragged fins or swimming on its side is alarming, and the usual reaction is to rush to the store for the first medication on the shelf. Nearly always it pays to stop for a moment: most aquarium fish diseases are opportunistic and show up when the water is poor or the fish is stressed. In this guide we go through the most common ones, how to recognize them, what usually triggers them and the first steps to take before adding anything to the water.

Why fish get sick

The parasites, bacteria and fungi behind most diseases are present in almost every aquarium. A healthy, well-kept fish holds them in check with its immune system and slime coat. Trouble starts when something weakens those defenses. The most frequent causes are:

• Poor water quality: ammonia or nitrite above 0, nitrate building up because of skipped water changes, or a tank that was never cycled.
• Sudden swings: in temperature, pH or hardness, for example when topping up with much colder tap water or moving fish without acclimating them.
• Stress: overstocking, aggressive tankmates, no hiding places, or schooling species kept alone.
• New fish without quarantine: the usual way ich and other parasites get in.
• Poor feeding: leftover food rotting in the tank or a monotonous diet.

That is why treating the symptom without fixing the cause so often leads to relapses.

Symptoms of the most common diseases

This table sums up the diseases you will see most often in freshwater tanks. Treat it as a guide: several symptoms look alike and one fish can have more than one problem at once.

Disease	Typical symptoms	Likely cause	First step
Ich (white spot)	White dots like grains of salt on the body and fins; fish flash or rub against decor	The parasite Ichthyophthirius, nearly always brought in by new fish or triggered by a temperature drop	Raise the temperature gradually and use a dedicated ich treatment
Fin rot	Ragged, whitish or reddish fin edges; fins get shorter	Opportunistic bacteria due to poor water or fin nipping	Test the water and do frequent water changes
Fungus	White or grayish cotton-like tufts, often on a wound	Injuries, dirty water, weakened fish	Check the water; isolate the fish if badly affected
Velvet	Fine gold or rust-colored dust, easiest to see with a flashlight; clamped fins, fast breathing	The parasite Oodinium, favored by stress	Dim the lights and start a dedicated treatment quickly
Dropsy	Swollen belly, scales sticking out like a pinecone, bulging eyes	A sign of internal failure, often after chronic stress or poor water	Move the fish to a hospital tank
Swim bladder problems	Swims sideways or head-down, floats, or cannot leave the bottom	Constipation, overfeeding, cold water or physical issues (common in round-bodied goldfish)	Fast for a couple of days and check the temperature
Columnaris	White or gray "saddle" patches on the back, cottony mouth, fast progression	The bacterium Flavobacterium columnare, closely linked to warm, dirty water	Water change, isolate and treat without delay

Ich is by far the most common disease and has its own guide: how to treat ich step by step.

First steps before medicating

Whatever the symptom, follow this order. It is the same for almost every disease and avoids half of all unnecessary treatments.

1. Test the water. Use a liquid test kit to check ammonia, nitrite, nitrate and pH. Ammonia and nitrite should read 0 and nitrate should stay below 25-40 ppm.
2. Check the temperature. A reliable thermometer rules out a faulty heater; for most tropical fish, 75-81 °F (24-27 °C) is normal.
3. Change the water: 25-30% with conditioner and water at a similar temperature. If ammonia or nitrite is high, repeat the next day.
4. Observe calmly. Is it one fish or several? Is it eating? Has anything new gone in over the last few weeks (fish, plants, decor)?
5. Isolate if needed. A hospital tank lets you treat a single fish without medicating the whole aquarium, its plants and its shrimp.

A hospital tank does not have to be expensive: a small 5-10 gallon (20-40 l) tank, a heater, a sponge filter seasoned in the main tank and a hiding spot are enough. Leave it bare-bottomed so it is easy to clean. The same setup doubles as a quarantine tank for new fish for 2-4 weeks, which is the best prevention there is.

When and how to medicate

Medication makes sense when the disease has been identified with reasonable confidence and does not improve after fixing the water, or when it moves fast, like velvet or columnaris. Some general rules:

• Use a product made for that specific disease, from established aquarium brands such as API, Seachem, Tetra or Fritz, and follow the label exactly: dose, duration and water changes.
• Remove activated carbon from the filter during treatment: it absorbs many medications and cancels them out.
• Work out the real water volume, minus substrate and decor, so you do not overdose.
• Increase aeration: many treatments and warmer water reduce the oxygen available.
• Be careful with shrimp, snails and sensitive fish: many products containing copper or dyes are toxic to invertebrates, and some scaleless fish, such as loaches or corydoras, tolerate certain treatments poorly.

Prevention: what actually works

Prevention is boring but effective. A cycled tank with weekly 20-30% water changes, stable temperature and sensible stocking has very few outbreaks. Add quarantine for new fish, careful acclimation and a varied diet in portions eaten within 2-3 minutes.

Choosing hardy species also helps when you are starting out. Fish like platies or zebra danios forgive small mistakes far better than delicate species such as the discus, which needs very stable, soft water. And if fish die without clear symptoms, see our guide on why aquarium fish die.

Common mistakes

• Medicating without testing the water: if ammonia is the problem, medication just adds more stress.
• Mixing several products at once: they can react with each other or add up in toxicity.
• Stopping treatment when symptoms vanish: ich and velvet have invisible life stages; finish the course on the label.
• Dosing the whole tank for one fish when a hospital tank is safer for plants, shrimp and beneficial bacteria.
• Leaving activated carbon in the filter during treatment.
• Adding new fish without quarantine to a healthy tank.
• Cranking up the temperature suddenly or doing huge water changes with cold water.

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