According to Bernice Brewster and Nick Fletcher's manual "Goldfish," these brightly coloured fish produce a lot of fecal matter, which can quickly throw off the chemical balance of the water. If there is too much ammonia and nitrates in the water, that greatly reduces the oxygen content that the fish need to breathe. Weekly partial water changes help keeps a goldfish tank healthy.
Determine a partial water change and algae cleaning schedule and stick to it. A general rule is that a goldfish tank needs a 10 - 20% partial water change every week. If the tank is overcrowded, then this needs to be done two more times a week.
Reserve one bucket for dirty water and one bucket for clean water. Fill the second bucket with room temperature tap water and either add dechlorinator or let sit for 24 hours before you clean the tank.
Scrub off algae from the tank sides and ornaments with the algae scraper or an algae cleaning brush. Only use the brush for that purpose. It may be easier to remove the ornaments from the tank in order to scrub them. If the tank threatens to overflow when you stick your hand in, then do this step after the next one.
Bring the bucket for dirty water, the siphon hose with the gravel cleaner at one end, over to the tank. Stick the gravel cleaner in the tank. On the other end, suck on the hose until the siphoning action starts, and then let the dirty water flow into the bucket.
Remove one fifth or less of the dirty water. As you do that, stick the gravel cleaner into the gravel to remove the dirt and excess algae (See resources). Do as much of the tank floor as you can without removing too much water.
Replace any cleaned ornaments.
Gently tip the bucket of clean water into the tank.
Replace any filtration media in the filter when the tank water gets cloudy.
Keep some algae in the tank every time you clean. Some algae not only helps to give the goldfish something to eat, but also helps to make oxygen for the tank.
Never overfeed your goldfish. This will make them urinate and excrete more, which can quickly raise the ammonia and nitrate levels to a dangerous range. Rotting food that the goldfish miss will also raise nitrogen levels. Never keep the lights on more than 12 hours a day, because that will encourage too much algae growth. Never clean the buckets, gravel cleaner, siphon hose, ornaments or algae scraper with soap. This clings to the object and can poison your goldfish when it comes in contact with the tank water.