Disease & Troubleshooting

Most aquarium problems are water quality problems wearing a costume. Cloudy water, algae outbreaks, sick fish, fish gasping at the surface, fish dying overnight — these are nearly always symptoms, not causes, and the cause is almost always something measurable in the water. The most useful skill in fishkeeping is not memorising medications; it is learning to read the symptoms back to the parameter that caused them. This section walks through the common problems and the root causes that actually fix them.

The single most important troubleshooting step is also the simplest: test your water. Before adding medication, before doing anything drastic, test ammonia, nitrite, nitrate, and pH. A spike in ammonia or nitrite is the cause behind most "mystery" fish deaths, and the fix is a large water change plus a look at whether your filter is undersized, recently washed in tap water, or simply not cycled. Skipping the test and reaching for medication kills fish more often than the original problem does, because medications stress already-stressed fish and can crash your biofilter.

Fish diseases fall into a few broad categories. Parasites (ich, velvet, flukes) cause visible spots, flashing against decor, and rapid breathing. Bacterial infections (fin rot, columnaris, ulcers) cause ragged fins, cottony patches, and lethargy. Fungal infections (white fuzz on wounds, mouth fungus) usually appear secondarily after a parasite or injury. The Common Aquarium Diseases guide covers the major ones with symptoms, causes, and step-by-step treatment. The nano fish diseases guide focuses on small tanks where diseases spread faster and treatment doses must be exact.

Algae is the other category of "problem" that drives beginners mad. The truth is that algae is not a disease — it is a plant, and it grows because conditions favour it. Brown diatom algae in a new tank is normal and disappears on its own within weeks. Green spot algae on the glass means low phosphate or low CO₂. Hair algae means too much light and too many nutrients in the wrong ratio. Black beard algae often means fluctuating CO₂ or organics building up. The algae guide explains each type and the specific fix — none of which involve buying an algae-eating fish, which almost never solves the underlying problem.

Cloudy water has three common causes: bacterial bloom (a sign of an immature or overloaded biofilter), green water (suspended unicellular algae, usually from excess light plus nutrients), and particulate cloud (insufficient mechanical filtration or disturbed substrate). Each has a different fix — bacterial blooms resolve with patience and reduced feeding, green water responds to blackouts and UV, and particulate clears with better filter floss. Treating the wrong cause wastes time and stresses the tank.

The experience guides in this section — Why Guppies Are Dying and The Truth About Endlers — are honest field reports from our own fishroom. They cover what actually happened when a species did not thrive, what we changed, and what we learned. More specific disease and troubleshooting pages are in progress (Cloudy Water, Green Water, Brown Algae, Hair Algae, Fish Gasping, Fish Dying Suddenly, Ich, Fin Rot, White Fungus) and will appear as coming-soon cards below.

A diagnostic order of operations

When something is wrong in your tank, work through checks in this order: parameters, behaviour, equipment, then disease. Test ammonia and nitrite first — if either reads above zero, that is almost certainly your problem and you need a large water change immediately, not medication. Next, observe fish behaviour for an hour: gasping at the surface means oxygen or ammonia, clamping fins means stress or parasites, flashing against decor means skin irritation or ich, hiding unusually means a bully or poor water.

After behaviour, check equipment. Is the filter running and not clogged? Is the heater holding temperature (verify with a separate thermometer, not the heater’s built-in dial)? Is the temperature within the species’s preferred range? Equipment failures cause a surprising number of "disease" outbreaks — a failed heater chilling a tank by eight degrees overnight will look exactly like a parasitic infection in the morning. A clogged filter that has stopped cycling water through its media will cause an ammonia spike that looks like fish illness.

Only after parameters, behaviour, and equipment are all ruled out should you reach for disease treatment. When you do, treat in a quarantine tank whenever possible — many medications crash biofilters, stain silicone, harm invertebrates, or kill plants. Copper-based treatments kill shrimp and snails outright. Malachite green (the active ingredient in many ich treatments) stains everything it touches. Quarantine tanks let you dose accurately, observe the sick fish in isolation, and protect the rest of your tank from collateral damage.

A final principle that took us years to internalise: euthanising a fish is sometimes the right answer. A fish that has stopped eating, lost buoyancy control, and shows no response to treatment over several days is suffering. A clove-oil overdose is the humane method. Holding out for a miracle recovery that will not come is not kindness; it is prolonging distress. Every experienced aquarist has made this call, and learning when to make it is part of responsible fishkeeping.