Fish can be expensive to purchase so it is important that you choose species that are compatible with each other and the type of aquarium that you own.

The choice of fish species now available as pets is enormous, both salt water and fresh water, so it is important to give some thought to the type of aquarium you require before you start purchasing the fish. Tropical fish fall into both salt water and fresh water categories, and require greater care with steady water temperatures and supplements. Aquarium retailers and pet shops may refer to tropical salt water fish as "marine fish". All fish species inhabit differing underwater environments, but can pretty much be put into three behavioural categories: community fish (such as gold fish), semi-aggressive fish, and aggressive fish. Semi-aggressive and aggressive fish (such as Siamese fighting fish) are territorial and will attack other fish,so don’t mix them. Once you have decided on the type of fish you want, consider how big they will grow. One or two typical gold fish can co-habit a small fish bowl, whereas a couple of shark catfish will need an aquarium the length of an average living room wall. Keep in mind that many fish like to rummage around for food, so always have a good quantity of gravel in your tank so they can "dig" with their mouths.

Fresh water aquariums

Choosing an appropriate aquarium for fresh water fish is straightforward. A cold water aquarium suits regular gold fish, and also tetras, guppies, barbs and danios. Most are easy to look after and do not need a heater (beware that some species may need a chiller!) but cold water fish tend to have a high "waste" output (which means more cleaning) and require highly oxyengenated water so you may need an extra filter or aerator. Many fresh water species fall into the community fish category and will get along famously with each other. Shop staff will advise you on which are most suited to be together. Tropical fresh water fish require an aquarium with an underwater heater, special lighting and filter. The equipment is mostly automated so you just need to monitor it daily, and do the usual weekly cleaning and water replenishment. Tropical fresh water fish are probably the most plentiful to choose from, and some have stunning colours, such as the tiny neons. If you want to get real fancy, consider a "brackish water" aquarium, which is a mix of salt and fresh water, as found in estuaries. These are similar to tropical fresh water tanks in set-up, but with a darker environment.

Salt water aquariums

There are three types of salt water aquariums: fish only, fish with "live rocks", and reef tanks. Fish only tanks are similar in set-up to tropical fresh water fish tanks, but you will need to buy a water test kit to monitor the pH, salinity, ammonia and nitrate levels. "Live rocks" means tanks that include rocks which harbour living organisms and good algae. These tanks need better lighting, and the live rock -such as Fiji rock – can cost about $600 for an average size tank. However, the rock provides the best natural form of biological filtration for a salt water aquarium. It is porous and contains organisms that rid the tank of nitrates. However the rock regularly requires supplements that sound like nuclear bomb ingredients: strontium, magnesium, iodine and calcium, among others. A reef tank nails it – imagine having a little piece of the Great Barrier Reef in your living room. These aquariums are designed for corals (which are living organisms), anemones, invertebrates, and the most beautiful tropical fish, such as parrot fish. Setting up and maintaining a reef aquarium is very tricky, because these inhabitants are obviously food for each other. They are all expensive, and the water parameters must be monitored daily. This involves water supplements, reverse osmosis, deionised water, perfect filtration methods such as live rock, and high levels of lighting. This all adds up to a lot of dollars, and may sound like one step short of building a new desalination plant, but it’s sure to impress your friends.

So, choose your fish and aquarium so they suit each other. Bear in mind how big the fish will grow – large angelfish can grow up to 60cm in length, and having just one or two requires at least 820 litres of water, whereas dwarf angelfish are only 10cm-15cm long and can survive in a moderately sized aquarium. Some fish get along better than others so make sure you place friends with friends, and enemies with … anemones.