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If your tap water is hard and alkaline, and a lot of ours is, the molly is close to the ideal fish. Most of the popular community species were bred for softer, more acidic water and only tolerate what comes out of my tap; the molly actively wants it. Warm, mineral-rich, slightly alkaline water is exactly the environment these fish evolved in, so for once the water that fights you with tetras and rasboras is working in your favour. Here is how I keep mine, and where people go wrong with them. ...

The ideal GH depends on the shrimp: Neocaridina, the common cherry shrimp and their colour forms, want a GH of roughly 6 to 12 dGH, while Caridina, the crystal reds and bee shrimp, want it far softer at around 4 to 6 dGH. GH matters more to shrimp than almost any other reading, because shrimp build a fresh calcium-rich exoskeleton every time they moult, and the water is where that calcium comes from. ...

For most tropical tanks in a normally heated room, size an aquarium heater at roughly 1 to 1.5 watts per litre. A 60 litre tank wants somewhere around a 75 watt heater, and a 100 litre tank a 100 to 150 watt one. That guide keeps the water at a stable tropical temperature without a heater that runs flat out or one that never keeps up. Free PDF ...

A general community tank is happiest at a stable pH anywhere between 6.5 and 8, and holding that number steady matters far more than hitting any exact figure. Almost every fish sold for a community aquarium is farm-bred and many generations removed from the wild, so it adapts comfortably to a wide band of pH as long as the water does not lurch about. A rock-steady 7.8 is far better for a shop-bought tetra than a tank that swings between 6.5 and 7.5 because someone keeps dosing it. ...

If you want to set up a tank for a betta properly, here is the whole thing in one sentence: a heated, filtered, cycled aquarium of around twenty litres or more, with a lid, live or silk plants and gentle flow. Everything below is the detail behind that sentence, in the order you actually do it. This is a setup guide, the shopping list and the build, rather than a care guide. I have written the day-to-day feeding, fin health and behaviour side separately in betta care; this piece gets the tank right in the first place, because a good setup prevents most of the problems the care guide then has to solve. ...

Water hardness is the part of fishkeeping that sends more beginners into a spin than anything else, and most of the panic comes from muddling two numbers that are not the same thing. GH and KH sit on every test strip, get written into every care sheet, and yet almost nobody explains what they actually measure or which one your fish care about. So here it is, without the fog. GH is the mineral content your fish feel through their bodies. KH is the buffer that keeps your pH steady. They move independently, they do different jobs, and once you can read both, water chemistry stops being a mystery and becomes one of the most useful tools you own. ...

If you have hard, alkaline tap water and you want a tiny cichlid that runs its own colony in a small tank, Neolamprologus similis deserves a place on your shortlist right next to its famous cousin. Similis is a shell-dwelling Tanganyikan, one of the smallest cichlids kept in the hobby, and it does everything the better-known multifasciatus does: it digs, it defends a shell, it breeds inside it and it builds a little society across the tank floor. The main practical difference is the look and the temperament, and both, to my eye, come out slightly in favour of similis. This is the species profile: what it is, how it differs from the multi, and how to keep and breed a colony in water like mine. ...

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. ...