There are three main points to remember about illness in the aquarium:
1. At least 95 per cent of illnesses are environmental rather than pathogenic (caused by an outside organism). Moreover many pathogenic diseases remain dormant unless the fish is weakened by environmental factors.
2. There is no point in medicating a fish unless you know exactly what is wrong.
3. Prevention is better than cure.
Point 1 is a bitter pill to swallow when you have tried to provide optimal conditions; many aquarists find it impossible to accept and continue to kill their fish while laying the blame elsewhere. A small closed ecosystem is, however, extremely vulnerable to imbalance or overload, even in experienced hands, so always suspect environmental causes (unless the pathogenic disease is obvious); even if water quality checks out OK an extra water change does no harm and often affects a cure.

Point 2 may sound obvious, but many aquarists panic when a fish falls ill, administering a succession of patent remedies in the hope of hitting the right one. The resulting chemical brew is more likely to poison the entire aquarium. Remember that fishes, like us, suffer organ failure, so a single corpse is a reason for concern, but not alarm. If more fishes become ill, and you cannot identify the cause, seek help from other more experienced aquarists or your vet, who should be able to arrange a post-mortem diagnosis. Few vets routinely offer this service, so discuss the possibility before the need arises.

Once a specific disease is diagnosed you can normally obtain the appropriate medication from a dealer, or, in the case of prescription drugs, the vet. Always follow any instructions to the letter, not only using the correct dosage, but completing the full course of treatment; otherwise you may destroy active pathogens, but not those waiting to hatch. Avoid medicating healthy fishes by treating noninfectious patients in a hospital tank (your quarantine tank in another guise) or by “spot treatment” (for example for fungus), after which the fish is returned to the community.
Remember that some medications, for example, copper-based treatments are toxic to some fish. Some may harm plants, marine invertebrates, and beneficial bacteria in your filter, and should, where possible, be used only in the hospital tank. Prevention (point 3) includes correct setting up, suitable maintenance, quarantine of all new arrivals, and constant observation of your fish.
These are only a few of the commonest problems here:

Symptoms: reduced appetite; minimal or no faeces; the fish may have a slightly swollen abdomen and/or rest on the bottom of the tank.
Cause: incorrect diet or over-feeding.
Treatment: half a level teaspoon of magnesium sulphate (Epsom salts) per 4.5 litres (1 gallon). If the fish recovers, improve the diet.

Dropsy/”Malawi Bloat”
Symptoms: swollen body, especially the abdomen, due to the build-up of fluid in the tissues.
Cause: environmental, or organ failure. “Malawi bloat”, which affects East African mouthbrooding cichlids, is caused by excessive salt (sodium chloride) levels, high nitrates, or an unsuitable diet.
Treatment: none; remedying the cause occasionally affects a cure.

Symptoms: fin membranes disintegrate and the rays become inflamed.
Cause: bacterial, triggered by poor water quality or injury to the fins.
Treatment: remedy the cause; spot treatment with gentian violet.

Symptoms: white fluffy growths on the body/fins.
Cause: fungus attacks areas where the protective mucous coating is damaged by injury, parasites, or the environment.
Treatment: remedy the cause; spot treatment with gentian violet, or use an aquarium fungicide for major outbreaks.

Gill Parasites
Symptoms: scratching, laboured respiration (both, however, normally symptoms of environmental problems); in severe infestations heightened colour, glazed eyes, and loss of motor control as oxygen shortage affects the brain.
Cause: usually flukes (Dactylogyrus).
Treatment: “Sterazin”, from aquatic dealers.
Hole in Head
Symptoms: white stringy faeces; sometimes enlarged, pus-filled sensory pores on the head. Affects mainly cichlids.
Cause: Hexamita, an internal parasite normally harmful only when the fish is weakened by other factors (age, stress, environment).
Treatment: metronidazole (“Flagyl”) or di-metronidazole (prescription drugs); 50 mg per 4.5 litres
(1 gallon) mixed with water before use; repeat after three days.

Intestinal Parasites
Symptoms: emaciation despite good appetite; worms sometimes protrude from the vent.
Cause: various species of intestinal worms.
Treatment: a suitable anthelminthic (from the vet) is administered in food.

Large Skin Parasites
Symptoms: scratching; individual parasites are visible on the skin.
Cause: fish lice (Argulus) are round and almost transparent, lying flat against the skin; anchor worms (Lernaea) are worm-like, attached at one end; leeches are also roughly worm-like but attached at both ends. All are more common in ponds than in aquaria.
Treatment: remove with tweezers, and apply antiseptic to the site. For serious pond infestations treat with 1.125 to 1.8 mg per 4.5 litres (1 gallon) of Metriphonate (an insecticide).

Pop-Eye (Exophthalmos)
Symptoms: the eye protrudes from its socket, which is inflamed.
Cause: usually environmental, rarely parasitic (which is incurable).
Treatment: correct water quality/chemistry. Recovery may take several days.

Skin Slime Disease
Symptoms: scratching/shimmying; a fine grey coating on the body/fins.
Cause: parasites of the genera Costia, Cyclochaeta, and/or Chilodonella; these generally attack only when the body mucus has been affected by the poor environment.
Treatment: use a proprietary remedy; correct water quality/chemistry.

Swim Bladder Disease
Symptoms: loss of balance, swimming upside down or on one side.
Cause: a) swim bladder bruised during handling, fighting, or breeding; b) bacterial infection, usually associated with poor water quality.
Treatment: transfer fish to shallow water; for b) improve water quality and treat the fish with a broad-spectrum antibiotic. If there is no improvement after a week, consider euthanasia.

Symptoms: a golden velvety coat on body/fins; scratching, increased respiration.
Cause: the parasite Oodinium.
Treatment: use a proprietary remedy.

White Spot
Symptoms: tiny white spots on body/fins; initially few, increasing dramatically after a few days.
Cause: the parasite Ichthyopthirius.
Treatment: use a proprietary remedy.
Finally, we suggest you do not keep a medicine chest. Buy medicines only when you need them, as you will then be less likely to panic dose unnecessarily or incorrectly.

Sooner or later you will need to destroy a fish which can no longer function properly as a result of illness or old age. The quickest and most humane method is to sever the spinal cord by cutting down hard just behind the head using a sharp knife. If you can’t face the task, consult your vet.

Equipment Failure
Keep spares of items which go wrong commonly and suddenly, for example, heater, air pump diaphragm. In the event of major failures, tank breakage, or filter breakdown, at unsociable hours, seek help from the fishkeeping friends you should have made (including your dealer).

Power Cuts
Notify the electricity company of the loss of supply and get an estimate of the duration. Keep tanks warm with blankets/quilts; plastic lemonade bottles, filled from the hot water tank, can be used as heaters. Use a battery-operated air pump or improvise aeration, for example, hourly short sessions with a bicycle pump. Filter bacteria may be affected, so reduce/stop feeding and monitor ammonia/nitrite for a few days.

Going on Holiday
Properly maintained set-ups need no special preparation. Ask someone to check the fish daily ideally an aquarist, but if not leave the phone number of a “trouble-shooter” in case of any problems with the fish or equipment. Most fishes will survive a fortnight without food and probably be healthier for it, but if you must have them fed, never leave a container of food. Other people are invariably over-generous but instead provide individually wrapped daily rations.