Look for fish that are bright and alert and free of obvious injury or disease. If possible, ask the shopkeeper to feed them, and only buy fish that accept the food. Avoid fish that look skinny or starved - even if they're feeding it's hard to bring them back to health.

Just like in nature, the quality of the water your fish live in is really important to their health. Only use clean water in your tank and add conditioner to tap water. You can get conditioner from your local aquarium shop. If you're using seawater, make sure it's not polluted. Frequent modest water changes (say one third every two weeks) are generally better than less frequent larger changes.

Clean the tank every week or so to make sure your fish are living in a healthy environment. Make sure all filters are working properly and clean if necessary (Note: If you have a seawater tank, you must clean your filters in seawater. You can use waste water from a water change for this.)

Check they're all eating properly and make sure each one gets enough food. This is especially important with the little ones. Look for problem signs like white specks on the body, missing scales or cuts and scratches.

Any changes in a particular fish's 'personality' should also be noted - for example if a normally active fish becomes shy and retiring, or if a usually well-mannered fish becomes aggressive and agitated.

If you notice anything wrong, you should first check the water, using available testing kits to check its condition. A water change of up to half the tank's volume can bring immediate relief in a crisis, but you'll still need to address the original cause of any water quality problems, for example overfeeding or a clogged filter.

Treatments are available for many common diseases but read the labels carefully and get advice if in doubt. Some treatments for sick fish may harm other things like plants, shrimps or corals.