You can wipe the the entire tank using 1 part white vinegar to 9 parts water. And then rinse it very well with hot water, you can use boiling water if your bowl is made of glass.
let it dry.
You can also boil the gravel. and other decorations that can be boiled.
hydrogen peroxide is also another option. The wash everything thoroughly. let dry for a while. leaving everything in the sun for a week is also a good idea :)
Make sure you acclimatize the new few fish properly. when selecting fish, try to go for the most active healthy looking (no fin rot etcetc). you can ask petland when their fish comes in and try to get your fish that day, before it goes through petshop abuse.
Here is a video of my fish. he is extra healthy. you can also view other youtube videos that might give you an idea of what a healthy etta should look like.
Bettas being lazy is a misconception. mine zooms around the tank all day from end to end, he's a complete ball of energy. (I'm flaring him because he was just overfed)