Water Chemistry

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. ...

My tap water comes out of the mains at somewhere between 22 and 23 degrees of German hardness, mostly calcium, with a pH of about 7.2 and even a little nitrate already in it, around 15 ppm before it ever touches a tank. By any honest definition that is very hard water. When I started keeping fish I assumed, as most beginners do, that this was a problem to be fixed, and that somewhere out there was a bottle or a filter that would turn my liquid limestone into an Amazon blackwater stream. It took me a while to realise the truth, which is that my water was never the problem. My stocking list was. Once I started choosing fish that actually come from hard, alkaline water, everything got easier. No chemicals, no remineralising, no weekly fight with test kits. The fish breed, the snails have glossy shells, and water changes are just water changes. This article is the one I wish I had read at the start. ...

Every tank I have ever kept has taught me the same lesson eventually: the water is the pet. The fish are just the visible part of a system that lives or dies on chemistry you cannot see. When something goes wrong in an aquarium, the answer is almost never in the fish themselves. It is in the numbers. The good news is that there are only a handful of numbers that matter, and once you understand what each one is actually telling you, testing stops being a chore and starts being a conversation with your tank. Here is what each parameter means, what a healthy reading looks like, and what to do when a reading is off. ...