The beginner's story usually goes like this: a tank is bought, filled with water, and fish arrive the same day. A week or two later the fish die one after another, the water clouds and smells. The shop is not to blame, and neither are "weak fish". The start-up is.
What happens in a new tank
Fish excrete ammonia. Ammonia is toxic to them.
An established tank contains bacteria that convert ammonia into nitrite (also toxic), and others that convert nitrite into nitrate, which is far safer and is removed by water changes. This is the nitrogen cycle.
A new tank has none of those bacteria. They appear on their own, but they need time — weeks rather than days. All that time, fish live in their own poison.
How to start
**1. Set the tank up completely.** Substrate, filter, heater, plants, light. The filter runs around the clock from day one and is never switched off — the bacteria live inside it.
**2. Give the bacteria something to feed on.** Without a source of ammonia there is nothing for them to grow on. The kindest method is a fishless cycle: add an ammonia source (a dedicated product or a pinch of food) and wait.
**3. Wait and measure.** You need tests for ammonia, nitrite and nitrate — liquid kits or strips. The cycle is finished when ammonia and nitrite read zero and nitrate is present. That usually takes two to six weeks.
**4. Stock gradually.** A few hardy fish first, the next ones two or three weeks later. A full group added at once produces a burst of ammonia that a young filter cannot handle.
Mature filter media from a running tank speeds the process up considerably.
Tap water
Chlorine from the tap kills bacteria and harms fish. Treat the water with a conditioner (dechlorinator) — an inexpensive product to buy along with the tank.
Letting water stand for a few days works partly against chlorine but not against chloramine; a conditioner is more reliable.
How many fish fit
Fewer than you would like. What matters is not how many fit, but how much water dilutes their waste.
A goldfish is not a fish for a round bowl: it grows large, eats a lot and produces a lot of waste, and needs tens of litres and good filtration. In a small bowl it lives a year instead of ten or twenty.
A betta is not a "no-maintenance fish" either: it needs heating and a reasonable volume.
Weekly routine
Change part of the water — commonly around a quarter of the volume, fresh water at the same temperature, treated with conditioner.
Never rinse the filter under the tap: hot water and chlorine kill the bacteria. Squeeze the sponge in the tank water you have just drained.
Feed small amounts — as much as the fish finish in a couple of minutes. Uneaten food is the main source of ammonia and cloudiness.
If the fish are already in and struggling
Fish at the surface, breathing fast, lying on the bottom, cloudy water. The first steps are frequent small water changes and no feeding for a day or two: fish are fine without food, and ammonia falls.
After that you need test results: without them, treatment is guesswork.
An aquarium is the one pet where the water matters more than the food. Get the water right once and nine out of ten future problems disappear.