Should I Keep Tropical Fish or Cold Water Fish?
Newcomers to the fish keeping hobby will often mistakenly think that it is better to keep cold water fish such as goldfish because they don't require a heater, and the fact that they don't require a heater seems to imply that they are easier to keep than tropical fish.
The opposite is true. Yes, tropical fish will require that their water is kept somewhere between 20 - 25 degrees Celsius depending on the species, but tropical fish are almost certainly a better bet for the beginner. Why is this? There are several reasons.
The major reason why I would recommend tropical fish for a beginner is the fact that cold water fish need much more room than tropical fish. A 20 gallon tank is the recommended minimum for just one goldfish, whereas in a tropical set up with adequate filtration, you could have up to three or four medium sized fish such as Angelfish and a few little shoaling ones like neon tetras in the same sized tank.
Cold water fish are better kept in an outdoor pond than an indoor one, though you can always raise gold fish and koi indoors for the first few years of life and transfer them over to a pond when they are of sufficient size. If allowed to grow to their full size, even a garden variety gold fish will reach 12 inches in length. Common goldfish are the common goldfish, the Comet, the Shubunkin, the Lionhead, the Oranda, the Veiltal, and the Fantail. These are undeniably beautiful fish, but it is cruel to keep them in tanks which stunt their growth.
All fish types will require adequate filtration, usually in the form of an external or internal filter. Under gravel filters are being phased out by many fish keeping stores as they are viewed as being less effective than modern canister, hang on back, and internal filters. Cold water fish such as goldfish are very messy fish and will require excellent filtration if their water is to stay clean.
Clean water isn't just an aesthetic consideration, it is a requirement for healthy fish. Fish wastes are high in ammonia, which is extremely toxic to fish. A good filter system will contain bacteria which convert ammonia into nitrates, and nitrites into nitrates. When setting up a tank for the first time, you will need to cycle your tank, preferably without fish, to build up these bacteria before you put fish in the tank.
All fish will also need water that is pH appropriate. Goldfish tend to prefer a neutral pH, while tropical fish have a wider range of pH requirements depending on species.
There are a much wider range of tropical fish species available to the aquarist, and though they do all have their own particular traits and needs, if you are unwilling to go to the effort to monitor water quality, do frequent water changes, and familiarize yourself with the needs of your fish, then any fish is probably not suitable for you.
Fish used to be sold as easy care pets, but the reality is that to keep them well, and keep them healthy, they take a great deal more care than your average cat. If you are getting a fish for a child, then be aware that the costs associated with it may actually outweigh those associated with your average family moggy, who simply needs good food, a few vaccinations, flea and worm care, and a warm lap. Fish require a decent sized tank, (not one of those little pet store sold things which are too small to house anything but a few tiny neon tetras or be used as hospital tanks,) a filter, heater, air pump, water testing kits, substrate, plants or plant substitutes, décor for fish to hide in, and a whole lot of love and attention.
Fish keeping is an incredibly rewarding hobby, but it also represents a not insignificant investment of time and money.