how do you clean a fish tank?

drain all the water then try scrubbing it.

It’s probably calcium buildup from the water, or dead algea, mostly likely calcium. For a full clean, Drain part of the tank into a big tall bucket, put the fish in the bucket and add your air supply to it to prevent the fish from dying of lack of oxygen. Don’t forget to put something on top of the bucket to prevent the fish from trying to jump out. Drain the rest of the tank. Remove the rocks, put them in a seperate large bucket filled with hot water and a capful of bleach, same with all decorations. Let soak for a while. Then get a calcium cleaner liquid from the pet store (can kill fish, hence doing a complete tank breakdown), and scrub the hell out of the edges of the tank to get all the crap off. Take a wet washcloth with some clean water (no soap) and clean the inside and outside of the tank really good. Rinse, rinse, rinse, the tank, etc. Then the same for the rocks and decorations, rinse, rinse, rinse. Make sure to get all the bleach out, bleach can kill fish. Use a strainer for the rocks, it helps get some of the bigger crap out of the rocks. Then clean the filters, use a long wire brush to get inside the pipes, replace/clean your filter, breakdown the rest of the filter, clean and scrub the inside and outside of that too. Put everything back together, and fill with water (use room temp water), don’t fill completely, bucket water will be added back into tank. Add whatever proper fish tank chemicals you need to add at that time and turn on the filters. Wait a few hours, make sure the temperature in the bucket and in the tank are the same, then slowly insert fish back into their tank. Don’t do it quickly, especially with hot or cold water, fish are temperature sensitive and can go into shock and die very easily if you are not careful. Then add the bucket water, again, do it slowly. Rework your rocks and decorations. Wipe down outside of tank with a little windex and a rag to clean it up a bit, clean up the mess you made from all the cleaning. And you’re done.

Don’t do a full tank breakdown all the time, maybe once every 4-6 months will suffice. Usually a quick scrub of the glass and edges, change the filters, vaccuum the rocks, and a 25%-40% water change, will do the trick once a week. There are chemicals available at the pet store to prevent calcium deposits that you add directly to the tank water, I don’t remember if they are harmful to fish or not, so follow the instructions on the bottle carefully, as well as all tank chemicals you use in the future.