Do a 75% water change and gravel clean the substrate once a week. If the plants are alive, you can gravel clean around them and just clean the open areas.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If you don't have a gravel cleaner, you can make one from a plastic 1 litre drink bottle and a garden hose, or you can buy one from a pet shop or online. If you buy a gravel cleaner, just get a basic model like the one in the following link.
Aquarium cleaning should take 30 minutes every two weeks. Learn how the professionals do it in 18 easy steps.
www.about-goldfish.com
To make a gravel cleaner, cut the bottom off a 1 litre plastic bottle and throw the bottom bit in the recycling.
Remove the lid and plastic ring from the top and throw those 2 bits in the recycling bin.
Put a garden hose in the top of the bottle and run the hose out the door or into a bucket.
To use the gravel cleaner, put the bottle in the tank and let it fill with water. Suck on the end of the hose and get the syphon going.
Push the bottle into the gravel and lift it up. The gunk in the gravel will get sucked out with some dirty water. Move the gravel cleaner to a different spot and push it into the gravel and lift it up. Continue doing this until you have drained about 75% of the tank water out. Stop gravel cleaning and refill the tank using water that has been dechlorinated.
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It's fine and normal for the filter media/ material to get dirty. Every couple of weeks, take the filter media and squeeze it out in a bucket of tank water. Then re-use the media in the filter. Tip the bucket of dirty water on the lawn.
You don't want to replace the media because it will contain good bacteria that keep the water free of ammonia and nitrite.
