When fishy wastes decompose they become ammonia - which is lethal to fish.

Your main aim as a fish keeper is to maintain ideal water conditions, and to do this you will have to grow a colony of filter bacteria - containing 2 types - which will break down the ammonia into nitrite, and then break that down to nitrates.

The third stage is where you and a designated fish-water only bucket come in.

NB: nitrates are not dangerous at low levels, but medium to high levels will stress your fish and a stressed fish is open to infection and illness. When removing water as described below, you are preventing a build-up of nitrates by diluting old water with fresh water.

Before you start removing water, check that you have a good supply of dechlorinator. Get a decent one that also neutralises heavy metals in the water.

Using a measuring jug, find out where the ten-litre mark is in your bucket and make a clearly visible mark there - many dechlorinators suggest a certain amount per 10 litres, and this comes in handy. Also get a new syringe (a 5 ml one should be fine), minus the needle, to measure the actual amount of dechlorinator you are adding to the replacement tap water in your bucket.

A fish-only water jug will ease getting the replacement water back into the tank - keep it with your bucket when finished.

Note: keep all equipment you have bought for your fish, ONLY for that. Consider marking the bucket "Fish Only" or similar.

Assuming you have stuck to my suggestion regarding amount of fish per 50 litres, and assuming daily feeding in controlled amounts, you will need to remove 25 per cent of the water every two weeks.

To measure this amount, simply put a ruler up the vertical side of your tank and find the halfway mark between the gravel and the water surface and make a mark. Then measure from this mid-point to the water surface and find the halfway point for this top section. Make a clearly visible mark there too - this point is your 25 per cent mark, and you will be aiming to drain the water out of the tank until it reaches the 25 per cent mark.

You will need to purchase a piece of tube to drain the water - you might want to get a U-bend to attach to this which will help keep the tube in the tank - you can hook it on the side of the tank.
An alternative is the standard gravel vaccuum, which has the added benefit of removing decomposing food and fishy wastes before they add to the filtration load.

When draining the water out, ideally drain it into your bucket and use that water to rinse out your filter media - this will minimise the loss of filtration bacteria and keep the ammonia-nitrite-nitrate cycle stable.

Once you have removed the 25 per cent tank water (you might want to use this to water plants - a nice gentle fertiliser), fill the bucket to the ten-litre mark, add the specified dechlorinator and swirl it around. Leave for a couple of minutes, then pour the treated tapwater into the tank. Repeat the process until the water level is restored.