Some fish demand careful water and a gentle hand. The zebra danio is not one of them. It is a small, striped, endlessly busy little thing that behaves as though it has drunk far too much coffee, and it is about as forgiving a fish as exists in the hobby. If you have hard tap water and want a lively shoal that adds movement to the upper half of a tank, danios deserve a serious look. I have kept them as a dither shoal in community tanks for years, and they have shrugged off conditions that would have finished more delicate species. ...

If you have hard, alkaline tap water and you want a fish that looks like a boiled sweet and behaves like it has somewhere to be, the platy is hard to beat. It sits alongside the guppy as one of the two livebearers I recommend most often to people starting out with water like mine, and in some ways I prefer it. Platies are a touch chunkier, a little calmer, and they come in colours that seem almost unreasonable for a fish you can buy for the price of a coffee. I have kept them for years, usually as the middle layer of a community tank, and they ask for very little in return. ...

Sooner or later most fishkeepers meet white spot, and the first time is a fright. You look at a fish one evening and it is dusted with tiny white grains, as if someone had shaken salt over it. That is ich, short for Ichthyophthirius multifiliis, and it is probably the most common parasitic disease in freshwater tanks. The good news is that it is very treatable if you understand what you are dealing with. The bad news is that people lose fish to it constantly, almost always because they stopped treating too soon. I have cleared it out of my own tanks more than once, and the whole thing comes down to respecting the parasite’s lifecycle. ...

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. ...