Its recommended by most hobbyists to replace around 30% volume of water in a freshwater aquarium on a regular basis. I usually do it fortnightly. Replace more than that can create problems like fungus and high levels of ammonia. Its not a hard as many people think and cleaning a medium sized aquarium can be done in an hour's time.
1. Siphon out the water and dirt on the tank bed; for this I use a siphon pump and suck out the dirt much like using a vacuum cleaner.
2. Clean the tank ornaments and the sediments beneath them
3. Replace the sponge element in the filter
4. Wash the activated charcoal layer in the filter
5. Add some water cleaner/conditioner , commonly called "the blue stuff".
you can find their products here [link]
Thats it !














