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With shell-dwellers the substrate is not a design choice, it is the fish’s home. They dig in it, bury their shells in it, sift it through their gills and draw territory lines across it. Get the sand right and the colony settles fast. Get it wrong, too coarse, too shallow, the wrong colour, and they stay skittish and off-colour for weeks. I learned this by getting it wrong first, so here is what actually works in my hard-water shellie tanks, and why. ...

If your tap runs hard and alkaline, a shell-dweller colony is the easiest brilliant tank you will ever keep. Give them a shallow, wide footprint, a good depth of fine sand, a pile of empty snail shells and water left more or less as it comes out of my sort of tap, and they get on with living. Mine spawn in a tank I barely interfere with. I have fought soft-water species for years and lost; shellies were the first tank where the water was already right and I just had to stop meddling. Here is how I build one. ...

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. ...

If your tap water is hard and alkaline, livebearers are the easiest win in the whole hobby. Hard water is not something they tolerate, it is what they evolved for, so the question is never whether a livebearer will cope. It is which one suits your tank. My tap runs at roughly 22 to 23 dGH with a pH around 7.2 to 7.4, which is very hard by any measure, and the livebearers in my tanks colour up and breed without me touching the water chemistry at all. That is the short version. The longer version depends on how big your tank is, how much breeding you can stomach, and how hardy you need the fish to be straight out of the bag. ...

If you are looking at a tuft of white or greyish cotton-wool growth on a fish, you are almost certainly looking at Saprolegnia, the water mould hobbyists just call fungus. The first thing worth saying is that it rarely attacks a healthy fish out of nowhere. Fungus is an opportunist. It settles on tissue that is already damaged, dead or weakened, which means the fluffy growth is a symptom of an earlier problem as much as a disease in its own right. Treat only the fuzz and it tends to come back; find out why the fish was vulnerable and you usually stop it for good. I have dealt with it a fair few times over the years, most often on a fish that had been nipped, chilled or kept in water I had let slide, and the pattern is always the same. ...

If your shell-dwellers have settled in, claimed their shells and still show no sign of fry, the usual cause is not something exotic. In nearly every stalled colony I have been asked about, it comes down to one of five things: the fish are too young, you have the wrong mix of sexes, there are not enough shells, the sand is too shallow, or something is stressing them. The good news for anyone on hard tap water is that the one variable people panic about most, water chemistry, is almost certainly already in your favour. Shell-dwellers come from Lake Tanganyika, and that lake is harder and more alkaline than most of our taps. My multies spawn in water straight from the cold line at about 22 to 23 dGH, and they could not care less. ...

Most freshwater aquariums do not strictly need a separate air pump. If your filter returns water so that it breaks and ripples the surface, you already have the gas exchange an air pump would give you, and the fish are fine. That said, there are real situations where a pump is either essential or a sensible bit of insurance, and there is one warning sign where you need more aeration immediately. I run a mix of tanks on hard water, some with pumps and most without, and the deciding factor is almost never the one people expect. Let me explain what an air pump is actually for, because once that clicks the “do I need one” question mostly answers itself. ...

A dither fish is an active, visible fish you add high in the tank to tell the nervous fish below that no predator is about, so they stop hiding and start behaving normally. That is the whole idea. In a cichlid tank it does a second job too, giving a bully something to chase that is fast enough not to get hurt, which takes the heat off the other cichlids. Whether you actually need one depends entirely on which cichlids you keep, and in my experience plenty of hard-water setups do fine without any. I have run shell-dweller colonies for years with nothing swimming above them, and run others with a shoal up top that changed the whole character of the tank. Here is how to tell which situation you are in. ...

The short answer is that you can, but most of the time you should not, and the reason has nothing to do with water. People ask me this all the time because both lakes give you hard, alkaline, colourful African cichlids, and it seems obvious they should share a tank. The water chemistry really does line up. Everything else pulls them apart. I have kept both lakes, in separate tanks, for years, and this is the honest version of why that separation exists. ...

If you keep a Tanganyikan tank on hard tap water and you want a catfish that actually belongs in it, Synodontis petricola is the one I reach for. It is a small, spotted, whiskery bottom-dweller from Lake Tanganyika, it stays sociable in a group, and it wants the same mineral-heavy, alkaline water that makes my shell-dwellers thrive. I have kept a little shoal of them under a colony of multies for years, and they are one of the least demanding fish in the house once you get two things right: the water and the diet. ...