One of the most common mistakes of beginner aquarists is to change aquarium water only once it looks dirty. After all, they have installed a mechanical filter. Unfortunately, filters can only remove physical dirt. They cannot remove ammonia build-up that is toxic to fish. Consistent water quality is the key to a healthy aquarium, and this can only be achieved through regular water change. Regular water change is not optional; it is an imperative - even when you have a skimmer and filter installed.

The recommended schedule for water change is once every 4 weeks. If you have plenty of fish or live plants in the tank, you may need to change more often since these organisms produce more waste. Test the water regularly to check for ammonia or nitrate buildup and see if there’s a need to introduce fresh water into the tank. 

Water change should be not be confused with cleanup. This another common mistake beginner aquarists make - they think they need to scoop out the fishes, drain all the water, give the tank a good scrubbing, then assemble everything right back in. To maintain water quality, one needs only to change a small percentage of the aquarium water. For freshwater aquarium, change 10% of the water once a month or as needed - according to test results. For saltwater aquarium, change 15% -25% of the water once a month or as needed. One-tenth to one-fourth of water change is enough to eliminate ammonia buildup but not enough to shock the fish.      

Before changing the water, it is recommended that you scrape the algae so the siphon can suck them.