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Fishkeeping is really water-keeping. The tank is not the pet; the water is. Fish excrete ammonia, which is toxic at almost any level you can measure. One colony of beneficial bacteria converts that ammonia into nitrite, which is also toxic, and a second colony converts nitrite into nitrate, which is comparatively harmless and which you remove by changing water. Growing those two colonies is called cycling the tank, and it has to happen before a single fish goes in. It takes four to six weeks — weeks, not days. Fish dropped into an uncycled tank suffer chemical burns to their gills and die in batches; the hobby calls this new tank syndrome, and it kills more pet fish than every disease combined. A liquid test kit for ammonia, nitrite, nitrate and pH is not optional equipment. It is the only way to see what is killing your fish. Cycled means ammonia zero, nitrite zero, nitrate rising. Then comes the routine that keeps it that way: a partial water change of roughly 25 to 30 percent every week, using temperature-matched, dechlorinated water, vacuuming the substrate as you go. Chlorine and chloramine from the tap kill the same bacteria you spent six weeks growing, so rinse filter media in old tank water, never under the tap. Stock by adult size, bioload, swimming style and temperament. The old one-inch-per-gallon rule is nonsense: it claims a fat, messy, oxygen-hungry 10 cm goldfish equals ten 1 cm neon tetras. Look up the adult length of every fish before you buy it — the common pleco is sold at 5 cm and reaches 45 cm and more. Schooling fish are not decorations. Most tetras, barbs, rasboras and danios are shoaling species whose whole nervous system is calibrated to the fish beside them. Alone or in pairs they are chronically stressed, washed out and often nippy. Six is the floor; ten or more is what they actually want. Buying two because two is what fits the budget is a welfare failure, not a compromise. Bigger is easier, not harder. A large volume dilutes mistakes and changes slowly; a 10-litre nano swings temperature and chemistry within hours, and a small swing kills fast. Sixty litres (about 15 US gallons) is an honest beginner minimum, and 100 to 200 litres (25 to 50 gallons) is far more forgiving. Most tropical community fish want 24 to 26°C (75 to 79°F) and a stable pH between 6.5 and 7.5 — stability matters more than any particular number. Our catalog follows the hobby's classic groups: livebearers, tetras, cichlids, labyrinth fish, catfish, coldwater fish, barbs and the wonderful oddballs. Read the group before you fall for the fish.