HOW OFTEN SHOULD I CHANGE THE WATER?

Change about a quarter to a third of the water once a week. Use a dechlorinator, also known as a water conditioner, if you are using tap water. Even rain water from a water butt can be polluted nowadays. When you are cleaning the filter, don't clean it in tap water but in the old water. Tap water kills the bacteria and upsets the balance of the tank. If there is a lot of debris on the gravel, think about buying a gravel cleaner. Most of all make sure the tank has a filter.

HOW DO I KEEP THE TANK GLASS CLEAN?

Algae in the water or on the tank sides flourishes where there is light and nutrients. To reduce light, move the tank as far away as possible from windows. Buy or make an aquarium backdrop, which you put on the back of the tank, on the outside. If you have a light on the aquarium, switch this on for 8-10 hours (no more) each day. Make sure you are not over feeding. Rotting food provides nutrients for the algae. Weekly water changing will help too (see above).