Start with the space, not the fish
It is tempting to pick the fish first. It works better the other way round. Measure the spot you have in mind, including the height available above it, and check that whatever it stands on can carry the weight. Water is heavy: a filled 100 litre tank and its substrate will weigh well over 100 kilograms.
Bigger tanks are more forgiving than small ones. Water conditions change more slowly in a larger volume, which gives you more time to notice and fix a problem. If you are choosing between two sizes and both will fit, the larger one is usually the easier first tank.
What you will need alongside the tank
A tank on its own is not a setup. At minimum you will want a filter sized for the volume, a heater if you are keeping tropical fish, lighting, substrate, a thermometer, a water conditioner and a test kit. A net, a bucket kept only for the tank and something to siphon with will all get used from the first week.
Let it run before you add anything
A new tank needs time to establish the bacteria that process fish waste. Set everything up, get it running, and leave it. Adding fish to a tank that was filled yesterday is the single most common reason a first setup goes wrong.
How long this takes varies with the method you use and the products involved. Come and talk to us about it, and bring a water sample once things are under way so we can see how it is progressing.
Then stock slowly
Add a few fish, wait, test, and add a few more. Stocking a tank over several weeks gives the filter time to keep up. There is no prize for filling it on day one.
Before you come in
Bring the tank dimensions, a note of any equipment you already own, and a rough idea of what you would like to keep. That is enough for a useful conversation at the counter.