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If you have hard, alkaline tap water and you want a small, colourful fish that will not just tolerate it but genuinely prefer it, guppies are one of the easiest recommendations I can make. Poecilia reticulata is the fish that got most of us into the hobby, and there is a good reason it keeps turning up in beginner tanks: it is tough, it breeds without any encouragement, and the males look like someone spilled paint. I have kept them on and off for years, and the tank in my kitchen has run more or less on autopilot for the last two. ...

It is the question every new keeper asks, and the honest answer annoys people because it is not a number. How many fish you can keep depends on which fish, how big they get, the shape of the tank, the filter you run and how often you are prepared to pick up a bucket. What I can do is give you a way of thinking that keeps you out of trouble, and steer you away from the rule that causes most of the overstocked, struggling tanks I get asked to rescue. ...

Most fishkeeping problems, once you dig into them, turn out to be the nitrogen cycle behaving in a way the owner did not expect. It is the single piece of biology that runs your tank, and it is completely invisible, which is why it trips up so many people. My fishless cycling guide covers the practical steps for getting a new tank ready. This piece is the companion to it: what is actually happening in there, and why each stage matters. Understand the cycle and a lot of the hobby stops feeling like guesswork. ...

There is a persistent worry among new keepers with hard tap water that plants are simply not going to work for them, that a lush green tank is something reserved for people with soft, acidic water and a cylinder of CO2 bubbling away. I had the same fear when I started, staring at water that measures around 22.5 dGH and assuming everything would sulk. It turned out to be almost entirely unfounded. The great majority of the plants sold in shops grow perfectly well in hard alkaline water, and several of the toughest actively prefer it. This is the list I would hand anyone setting up a low-tech planted tank on water like ours. ...

Losing a fish is miserable, and when it keeps happening it is easy to decide you are simply unlucky, or that the hobby is not for you. In almost every case I have been asked to help with, the fish were not weak and the shop was not selling duds. Something in the tank or the routine was quietly wrong, and once it was found the deaths stopped. This is the checklist I work through with people, roughly in the order the problems tend to show up. ...

The first fish you keep matter more than almost anything else you buy. Get a hardy, forgiving species and the inevitable early wobbles, a missed water change, a slightly overdue filter clean, a parameter you did not quite understand, become survivable lessons. Pick something delicate and those same mistakes turn into a row of little bodies and a beginner who quietly decides the hobby is not for them. I have watched both stories play out many times, and the difference usually came down to the fish on that first shopping trip. So this is the list I give friends who are setting up their first tank, with a particular eye on the sort of water most of us actually have coming out of the tap. ...

You walk past the tank one morning and the water has gone the colour of pea soup. The fish are still swimming about quite happily, the equipment is running, nothing smells wrong, and yet you can barely see the back glass. It is one of the more alarming things a new tank can do, mostly because it seems to arrive overnight and no amount of wiping the glass makes any difference. I have had it happen on new setups, on tanks that sat too near a window, and once on a tub of fry I was raising outdoors. The good news is that green water is one of the easier problems to fix once you understand what it actually is, and it very rarely hurts the fish. ...

I am old enough to have run under-gravel filters when they were just what a filter was. A slotted plastic plate under the gravel, a couple of lift tubes, an air pump chugging away in the corner, and a tank of guppies that outlived the family cat. Then the hobby moved on. Canisters arrived, hang-on-backs got cheap, and somewhere along the way the under-gravel filter went from standard kit to the thing you are gently warned against by every shop assistant and half of YouTube. “Nitrate factory.” “Anaerobic dead zones.” “Antiquated.” I nodded along for years. ...

The tank I look at most often is not my biggest or my prettiest. It is a low, wide box sitting on a shelf in the bathroom, holding maybe fifty litres of very hard water and a floor covered in empty snail shells. Inside it, a colony of the smallest cichlid in the world runs its own affairs with almost no help from me. These are shell-dwelling cichlids, Neolamprologus multifasciatus, and after several years of keeping them I am still not bored of watching. This is the practical account of how I keep the colony, what my hard tap water does for it, and the handful of things I got wrong before I got them right. If you want the full species profile, I have written one already on Neolamprologus multifasciatus; this piece is the lived version, the day to day of actually running a shellie tank. ...

The colony in my bathroom tank started as six fish. Nobody asked my permission for what happened next, and that is rather the point of Neolamprologus multifasciatus. These are shell-dwelling cichlids from Lake Tanganyika, usually just called multies, and they hold a genuine claim to being the smallest cichlid in the world. A big male is 4 to 5 cm, females noticeably smaller, and yet they carry all the character of their much larger relatives. They dig, they squabble, they defend, they raise young, and they do it all in a tank that fits on a sturdy shelf. If you have hard, alkaline tap water and you want your first taste of Tanganyikan cichlids without a 300 litre commitment, I honestly cannot think of a better fish. ...