Water: testing, changing, and what the numbers mean.

The weekly routine

Once a week, take out 20 to 30 percent of the water and replace it with fresh tap water that has been treated with dechlorinator and brought to roughly the same temperature. While the water is low, run a gravel siphon over the substrate to lift out waste. If the filter sponge is clogged, squeeze it out in the bucket of old water you've taken out, which keeps the filter bacteria happy. That's the whole job, and it takes about fifteen minutes.

The water change does three things at once: it removes nitrate, it tops up minerals the fish and plants have used, and it dilutes anything building up that you can't test for. Nothing else in the hobby gives back as much for as little effort.

What to test, and when

A liquid test kit for ammonia, nitrite, nitrate and pH covers everything a community tank needs. Strips are quicker and less accurate; they're fine for a rough check once the tank is settled.

• New tank, first two months: ammonia and nitrite every few days. Any reading above zero is your cue for a water change.
• Settled tank: nitrate and pH once a fortnight or so. Ammonia and nitrite only if something changes in the tank.
• After adding fish, after load shedding, or if a fish looks off: test everything before you do anything else.

What the numbers mean

Cape Town tap water

Two things are worth knowing. First, the municipal supply is treated with chlorine, so every bucket that goes into the tank needs a dose of dechlorinator. A capful per bucket is typical; check your bottle.

Second, Cape Town's water is generally soft, with low carbonate hardness. Soft water suits most of the fish we sell: tetras, rasboras, corydoras, angelfish, discus and the South American cichlids all come from soft water and are at their best in it. It also means pH can drift downward between water changes, which is one more reason to keep the weekly routine. If you keep fish from the African rift lakes, which want hard, alkaline water, ask us about a buffer or crushed coral in the filter to hold the pH up. If you're unsure what your tap water is doing, bring us a sample and we'll test it.

Temperature through the year

Tropical fish are happiest between 24 and 26 degrees, all year. An indoor tank in a Cape Town winter will drop well below that at night without a heater, so a heater is standard kit for anything tropical. Goldfish and other cold-water fish are the exception.

Summer is the other end. A tank in a warm room can climb past 30 degrees in a heatwave, which drives oxygen out of the water. Keep the lights off during the hottest part of the day, take the lid off or run a small fan across the surface, and add an air stone. A sealed bottle of frozen water floated in the tank will bring the temperature down in an emergency. For a room that runs hot every summer, a chiller is the permanent answer.

Small things that make a big difference

• Feed small amounts, and the water stays cleaner between changes.
• Keep the dechlorinator next to the bucket so it's always to hand.
• Write your readings down. A trend tells you more than any single test.
• If a reading surprises you, test again before acting on it. Kits aren't perfect, and neither is the lighting in most kitchens.