For plastic tanks, I remove the tank, empty it out, and plug all but the filler hole. I then rinse it with hot soapy water the best I can and check it with a flashlight. If there is still buildup present, I put a little aquarium gravel and hot soapy water in the tank and shake it to agitate the gravel, which usually loosens anything stuck to the tank. Once I'm satisfied it's clean, I remove the soapy water and gravel and rinse with hot water. Let it dry and inspect with a flashlight for cleanliness and to make certain there isn't a pebble stuck somewhere. That's about it. You can use solvents and such, but I don't like to. Metal tanks can be cleaned in a similar fashion, but if you have rust it gets exponentially more complicated. There are many videos out there on cleaning and coating metal tanks. You can get about as elaborate with that as your patience can stand. Sent from my moto g(6) using Tapatalk