If I could change one thing about how fish shops sell to beginners, it would be the little “starter kits”. The 20 litre cube with a light in the lid, the plastic bowl with a goldfish sticker on the box. They look manageable, they fit on a desk, and they are the single biggest reason new keepers give up in the first three months. I run a room full of tanks now, from 30 litre shell dweller boxes up to a proper community tank, and I will tell you plainly: the small ones are the hard ones. If you are buying your first aquarium, buy bigger than you think you want. Somewhere between 60 and 100 litres is the sweet spot, and here is why.
More water means more forgiveness
Every mistake a beginner makes ends up in the water. Feed too much and ammonia rises. Skip a water change and nitrate creeps up. A fish dies behind an ornament and rots for two days before you notice. In a 100 litre tank, each of those mistakes is diluted into 100 litres of water. In a 20 litre tank, the same mistake is five times more concentrated, and things go wrong five times faster.
Temperature works the same way. A large body of water heats and cools slowly. If your heater sticks on overnight, a big tank drifts up a degree or two before you spot it. A nano can cook. If the heater fails in winter, a 100 litre tank loses heat gently over many hours, while a small tank on a windowsill can drop below safe temperatures for tropical fish in an evening.
The chemistry is the same story. A larger volume holds its pH and hardness more steadily between water changes, which matters a great deal while you are still learning to read a test kit. If you have not yet read up on what those numbers mean, my guide to aquarium water parameters covers ammonia, nitrite, nitrate, GH and KH without the fog. And whatever size you buy, cycle it properly before fish go in. Fishless cycling costs you three or four weeks of patience and saves you from the classic new tank disaster.
None of this means a small tank cannot work. I keep nano tanks myself. But they work because I check them daily out of habit and I know what a problem looks like at a glance. That is exactly the experience a beginner does not have yet. Buy the forgiveness instead.
Footprint beats height every time
Two 100 litre tanks can be very different animals. One might be 80 cm long and 35 cm front to back. Another might be a tall column, 45 cm square and 60 cm high. On paper they hold the same water. In practice the long, low tank is better in almost every way.
Fish swim horizontally. A tetra or a rasbora uses length, not height. Territorial fish like dwarf cichlids carve up the floor of the tank, so the floor area is what sets how many territories fit. Gas exchange happens at the surface, so a bigger surface area means more oxygen. And when something goes wrong at the bottom of a 60 cm deep tank, you will be up to your armpit fishing it out.
When you compare tanks, look at the length and the front to back depth first, and treat the height as mostly decorative. A standard 80 x 35 x 40 cm tank of around 112 litres is, to my mind, close to the ideal first aquarium. It fits real fish, real plants and a proper filter, and it sits happily on furniture of a sensible size.
Glass or acrylic
For a first tank in the common sizes, buy glass. It is cheaper, it does not scratch when you scrape algae off it with a blade, and it stays clear for decades. Acrylic is lighter and stronger, which matters for very large tanks and odd shapes, but it scratches if you look at it wrong. One bit of gravel caught under an algae pad will leave a permanent arc across the front of an acrylic tank. Every glass tank I own still looks new after years of scraping. My one acrylic container is hazed with fine scratches and lives out of sight.
All in one kit or separates
The boxed kits with a tank, hood, light, filter and heater are convenient, and the better ones from established brands are perfectly usable. Their weak points are usually the filter, which tends to be small, and the light, which is often too dim for anything beyond the easiest plants.
Buying separates costs a little more effort and often no more money. You pick a tank, then a filter rated generously for the volume, then a heater, then a light suited to what you want to grow. The big advantage is that when one part disappoints you, you replace that part rather than working around it. Filtration is the piece worth getting right first, and I have written up how the options compare in my guide to aquarium filtration. If plants appeal to you, and they should because they make everything easier, start with the species in my list of beginner aquarium plants, most of which grow under quite ordinary lights.
If you do buy a kit, my advice is simple: treat the included filter as a starting point, not a ceiling, and be ready to upgrade the light later.
Where to put it
Water weighs one kilogram per litre. A 100 litre tank with glass, substrate and rock is comfortably over 130 kg, concentrated on a footprint smaller than a desk. That rules out flimsy flat pack furniture. Use a purpose built stand, a solid sideboard, or a properly braced bench. Check it is level with a spirit level before filling, and shim the stand rather than the tank. A tank that sits twisted on an uneven surface carries stress in the glass permanently, and seams do occasionally let go years later because of it.
Keep the tank out of direct sunlight. A window that throws sun onto the glass for a few hours a day will hand you green water and coated glass no matter how well you feed and clean. If you ever find yourself fighting a permanent green film, sunlight and overfeeding are the first two suspects, and my article on aquarium algae goes through the rest.
Practical siting points that get forgotten: you need a power socket nearby, ideally with a drip loop in every cable so water cannot run down into the plug. You need to be able to reach the tank with buckets or a hose, because water changes happen every week for the life of the tank. And you want it somewhere you actually sit, because a tank you see daily is a tank whose problems you catch early.
The bowls and tiny cubes
I will say it directly: do not buy a fish bowl, and do not buy anything under about 30 litres as a first tank. Bowls cannot be filtered or heated properly, their surface area is smallest exactly where it matters, and nothing sold in an average shop lives well in one. The tiny cubes are less bad but they are expert tanks in disguise, with fast chemistry swings and almost no stocking options. The trade sells them as easy because they are cheap. Cheap and easy are not the same thing.
A 60 to 100 litre glass tank, long rather than tall, on solid level furniture away from the window, cycled without fish and filtered generously. That is the boring, reliable recipe. Every experienced keeper I know started smaller than that, struggled, and wishes they had not.