Fish tank basics

Cycling the tank

Fish waste turns into ammonia. Bacteria convert ammonia to nitrite, and a second colony converts nitrite to far-less-toxic nitrate, which you remove with water changes. Growing those colonies is "cycling" and takes roughly four to eight weeks.

• Set up the tank, filter and heater and run them with no fish.
• Add an ammonia source — bottled ammonia, or a pinch of fish food daily.
• Test every couple of days and log the readings.
• The cycle is finished when ammonia and nitrite both read 0 and nitrate is climbing.

What to test, and what is safe

Use a liquid test kit rather than strips — strips drift badly as they age. Stability matters more than hitting a perfect number: a steady pH of 7.9 beats one that swings between 7.2 and 8.2.

Water changes

A weekly 20–30% change suits most stocked freshwater tanks; lightly stocked reefs often do 10–15% weekly. Always match temperature, and dechlorinate tap water before it goes in. If nitrate keeps climbing past your target, change more water or feed less — those are almost always the two causes.

Stocking sensibly

• Add fish slowly — a few at a time, weeks apart, so the bacteria keep up.
• Check adult size, not shop size. Plenty of "starter" fish outgrow small tanks.
• Schooling species need groups of six or more, or they stress and hide.
• Betta are solitary with fins others will nip — plan tankmates carefully.