Fresh water and a scrubbing brush/ sponge/ nylon scrubbie

You can use vinegar, salt or PLAIN diluted bleach, as long as you rinse well afterwards,

Never use soap or detergent, it toxic to fish and hard to get rid of all the residue.

The tanks dont have to be sterilised, they are going to grow more bacteria and algae as soon as you fill them up anyway. Just get them looking clean and your fish will be fine.

Ian