Water Chemistry

If you fill your tank from the mains, use a dechlorinator every time, no exceptions worth risking. That is the practical answer and it holds for nearly everyone. The water company adds chlorine, or increasingly chloramine, to keep the supply safe for people to drink, and both are harmful to fish and to the bacteria your filter depends on. A small bottle of conditioner costs very little and lasts months. A wiped-out filter or a tank of gasping fish costs you a great deal more. I have kept fish on hard tap water for years and I have never once skipped it on a water change. ...

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. ...

Walk into the cichlid section of any shop and you will find a shelf of tubs promising to turn your tap water into Lake Tanganyika or Lake Malawi. Buffers, cichlid salts, trace element blends. The useful thing to say first is this: if your tap water is genuinely hard and alkaline, you almost certainly do not need any of them. I keep Tanganyikan shell-dwellers on tap water that runs about 22 dGH, and I have never once reached for a tub of buffer. My fish breed, colour up and get old. The shelf is aimed at people whose water is soft, and if that is not you, you are being sold a fix for a problem you do not have. ...

Every few weeks someone tells me their tap water reads pH 8, they have panicked, and they are halfway through emptying a bottle of pH-down into the tank. So let me say the useful thing first: a high pH, on its own, is almost never the problem people think it is. A stable pH of 8 will keep a great many fish in fine health for years. What actually harms fish is a pH that lurches up and down, and chasing a high reading lower is one of the surest ways to cause exactly that lurch. I keep tanks on tap water that runs about 22 dGH and sits firmly on the alkaline side, and I stopped fighting it a long time ago. ...

Most fishkeepers do not need reverse osmosis water, and a good number who buy a unit end up barely using it. That is the honest starting point, and it goes against a lot of the advice you will read. RO strips your tap water down to almost nothing, minerals, buffering and all, so you can rebuild it to whatever you like. That is genuinely useful for a narrow set of fish and a narrow set of tap water. For everyone else it is an expensive, wasteful answer to a question they do not actually have. I keep hard-water fish on hard tap water, and my RO membrane spends most of the year dry. Let me explain how I decide, so you can work out whether you are in the group that needs it or the much larger group that does not. ...

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. ...

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. ...

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. ...