I decided to write this after reading a number of reviews on poor advice from pet stores on keeping fish. I have kept fish most of my life and used to be in charge of fish where I worked. Not that I was really an expert, but far wiser than the idiot who decided all the Siamese fighters should be together so they wouldn't be lonely, or the manager who thought a shipment of small cardinal tetras would make good tank mates for very large key hole cichlids. This will not be comprehensive or go into much detail on types of fish but will just try to include the basics.

CHOOSING A TANK

Try to buy the largest tank possible. A larger tank is much easier to maintain and will provide a more stable water quality. While many of the novelty tanks are lovely, they usually are not the best choice. They lack heating for small fish, and goldfish do grow big. They also pollute water quickly and are really better off in larger tank. The idea of fish growing to the size of the tank is not true. They will not reach the size of wild specimens usually, but fish often outgrow tanks, especially goldfish if they live long enough. My last goldfish lived 10 years in progressively larger tanks until I could no longer find room for yet a bigger tank. At 14" they were re homed to a pond owner. These were common 1 1/2" comets when I got them.

SETTING UP A TANK
This is the biggest are for mistakes and I slaughtered everything I bought for my first tank by listening to a pet shop attendant who didn't have a clue. They later tried feeding me a load of pseudo scientific nonsense, but to make things simple your tank needs to be very slowly stocked until the good bacteria build up. This is called cycling. The simple explanation is, fish poop and pee, nitrites and ammonia build up in water and poison fish. mechanical filtration does not remove these nasties. The only way to keep things in check is with the proper amount of good bacteria, which break the down the waste into non harmful substances.

There are two ways to cycle a tank, one is called a fish-less or chemical cycle and I do not know enough about it to explain it properly, but you can google it for more info. The other, most common way is by adding fish very slowly. Start out with only 1 or 2 hardy fish and wait for 8 weeks until the cycle is complete, and then add fish slowly. Of course this is more difficult with a very small tank because while one or two small catfish in a 2' tank is very understocked and will not build up too many chemicals. Two goldfish in a small tank is already heavily stocked. The ebst you can do is daily partial water changes with water aged at least 24 hours. DO NOT RINSE GRAVEL, this is where your bacteria is building up. You will have to rinse the filter frequently, but ideally this should be done in water drained from tank. Do not use hot water as this will kill bacteria.

Can you jump start this? Yes a bit. If you have more than one tank you can run the new filter in an existing tank for a week or so before setting up your new tank. This will build up some bacteria. You can also leave a nylon stocking full of gravel in an existing tank to develop bacteria, and even a good amount water from a healthy tank will help.

You can buy test kits to show if the water is within safe parameters. One type is reviewed on dooyoo "API 5 in 1 Aquarium Test Strips" if you want to look them up. I believe others have been as well, so if anyone has done a review can you post info in comments? Cheers.

BUYING FISH

Unless you are 100% certain you are dealing with a knowledgeable sales assistant, look up each species online to see if they are compatible. I once had a sales assistant try to sell me Oscars for a community tank with very small fish. Choose the one species you want most and work around that. For instance if you are set on neon tetras you must choose all relatively small peaceful fish. If you want piranha you can not have anything else! I would also check water requirements myself. i have in the past been sold fish as freshwater, only to find out that they require brackish water as they get older. Brackish water is a nightmare to maintain, and of course not suitable for many other fish. Check specialised dietary requirements as well. While most fish will do okay with just flake, occasional frozen foods are better for all of them. Some however can only eat frozen foods, and may even require special foods like cockles. If you are not sure you will be able to provide the correct diet, choose another fish.

Plectoctomus- One of the most common fish, this really needs a large tank, at least 3-4'. For a smaller tank choose ancistrus who looks much the same but will remain under 6".

Beware of dyed fish. This is a cruel process which causes a lot of pain and suffering and severely weakens the fish. Most will die within a month or two, but if they survive the bright colours will fade anyway. Parrot fish are commonly dyed. Parrot fish are naturally a white to light pink shade with a possible light golden tint around fins. This colouration can be enhanced by feeding brine shrimp, but if the fish is bright orange, strawberry coloured or blue it is dyed. Glass fish with neon stripes will also be dyed, or "painted". Many albino fish are also coloured now.

Also be aware of any fish with badly damaged fins, white spots or fungus.

MAINTENENCE

Once a tank is properly set up and cycled the maintenance is not that much. You simply part of the water every few weeks to every few months depending on type of fish and filtration. I prefer out of tank filter which require less maintenance, but it is up to you. Never take more than 1/3 of the water at a time, and never thoroughly clean your gravel and filter at the same time.

I have had my tank for years now and do very little with it. Some of my fish are now 10 years old. The tank and fish are well settled so all I do now beyond basic maintenance is sit back and watch them :)

Finally, please don't think of fish as disposable pets. I know we don't spare much thought for the ones served up with a plate of chips, but any animal purchased as a pet should be entitled to some standard of care.
If you find you can not keep them, try local fish shops or advertise on Gumtree. Keep in mind, with proper care many fish can live for a decade or more.