If its a second hand tank then I would first fill it up with warm water and pour in whole bottle of hydrogen paroxide 9% solution ( tanks 30 gal + ). You can get this at any Tesco Pharmacy. Let it sit for 1 hour, this will eliminate any live organisms that might survive other chemicals. At the end just flush it properly, but dont get paranoid, its not a poison.
Yes you can use tap water for cleaning, why not, just give it a flush with RO and wipe it dry. Its not like few drops of tap water would spike your PO4 and else. Dont worry.
For any dirt on glass, dry - limescale like stains i would recommend Steel Wool - Fine grade, nothing works better on glass and will remove pretty much everything without scratching it ( dont press it too hard - test it on other area first )
Anything more stubborn, just use razor blade, each glide you do, wipe of what came off the glass to prevent scratching with loose material.
Best of luck !