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If you have hard, alkaline tap water and you want a splash of solid yellow that is not a Malawi mbuna, the lemon cichlid is one of the best fish you can put in it. Neolamprologus leleupi is a small rock-dwelling cichlid from Lake Tanganyika, the colour of a ripe lemon from nose to tail, and it wants exactly the mineral-heavy water that comes out of my tap. I keep mine in a Tanganyikan tank alongside shell-dwellers, and once you understand its one real quirk, its temper with its own kind, it is a straightforward, long-lived fish. ...

If your guppies are healthy and you have both sexes, you will have fry whether you planned them or not. The good news is that guppy fry are about the easiest baby fish there is to raise, and if you keep them in hard, alkaline tap water like mine, you already have one of the main ingredients for fast, strong growth without buying a thing. This is how I bring guppy fry on from a surprise batch in the morning to young fish you can sex and move on, and the honest limits of what hard water can and cannot do for them. ...

If your tank is a few weeks old and a brown, dusty film is creeping over the glass, the sand and the leaves, take a breath. That is almost certainly a diatom bloom, and it is one of the most normal things a young aquarium does. Nearly every tank I have set up has gone through it. The good news is that it usually clears itself as the tank matures, the cleanup is easy, and you do not need to panic-buy anything. Let me show you how to be sure that is what you are looking at, why it happens, and what actually shortens it. ...

Driftwood will not rescue you from hard, alkaline water. I wish someone had told me that before I spent a weekend boiling a big piece of mopani and watching the pH in my test tube refuse to budge. The short version: driftwood releases tannins, tannins are mildly acidic, and in soft water with little buffering they can pull the pH down a touch. In water like my tap, which runs around 22 to 23 °dGH with a carbonate hardness to match, the buffering swamps the acid long before it reaches the fish. You get the tea-coloured tint and almost none of the pH drop. If that is the whole reason you are buying wood, save your money. ...

If you have hard, alkaline tap water and want something with real presence that is not a Rift Lake cichlid, the paradise fish deserves a look. It thrives across a huge range of hardness, sits comfortably in water like my 22 to 23 °dGH tap, and it is one of the few genuinely tropical-looking fish you can keep in an unheated room. The catch, and there is always a catch, is temperament. A male paradise fish is a bold, territorial little fighter, so this is a fish you plan the tank around rather than dropping into an existing community. Get that one decision right and it is among the most rewarding and undemanding fish I have kept. ...

If your test kit shows ammonia, act today, not at the weekend. The single most effective thing you can do is a big water change with clean, dechlorinated, temperature-matched water, assuming your tap reads lower than your tank. Everything else here is about buying time and stopping it happening again. And there is one detail the general guides skip that matters a great deal on water like mine: hard, alkaline tap makes the same ammonia reading far more toxic than it would be in soft, acidic water. If you keep fish on hard water, read the pH section below before you do anything else. ...

Tropheus are, on paper, the perfect fish for water like mine. They come from Lake Tanganyika, the hardest and most alkaline of the great Rift Lakes, so my 22 to 23 °dGH tap that soft-water fish hate is close to what they evolved in. They are stunning, they graze the rocks all day like little goats, and a settled colony in a big tank is one of the finest sights in the hobby. So why do I steer most people away from them? Because Tropheus punish two mistakes harder than almost any other cichlid: the wrong diet and the wrong group size. Get either wrong and you will not get a warning, you will get dead fish. ...

Nitrate is the last stop on the nitrogen cycle, the thing that builds up in the water once your filter is doing its job properly. It will not kill a fish overnight the way ammonia does, so it is easy to ignore, and that is exactly the trap. Let it climb and sit high for weeks and you get stunted growth, dull colour, more algae than you can scrub off, and fry or shrimp that quietly refuse to do well. The fastest way to bring it down is a water change, as long as your tap water holds less nitrate than your tank does. Everything else on this page is really about slowing how quickly it climbs back up again. ...

Gouramis are some of the most rewarding fish for a calm community tank, but the one everyone reaches for first, the dwarf gourami, is the one I most often talk people out of. On hard alkaline water like mine, at 22 to 23 °dGH, the choice of species matters twice over: once for how the fish handles the chemistry, and once for how likely it is to arrive already carrying something it will not recover from. Get the species right and gouramis are hardy, characterful and long-lived. Get it wrong and you are in for a slow, frustrating loss. ...

Yes, you can keep corydoras in hard water, but only some of them, and the honest picture is more nuanced than the forums usually let on. On my tap, which runs around 22 to 23 °dGH with a pH near 7.2, I have kept bronze and albino cories for years with no trouble at all. I have also watched more delicate wild-type species sulk and fade in the same water. The species you choose matters far more than any bottle of buffer you might be tempted to buy. ...