how to clean an old out of use tank?

I would use bleach water. If you have any hard water stains on it, you can use vinegar and a razor to get those off. After you clean it make sure you rinse very very well then I would rinse again with dechorinated water. Then you should be good to go.

Here's what I did:

First off I filled it up with water and left it overnight to see if there were any leaks. There weren't.

I then scrubbed the tank with hot water and brushes/towels to get all the gunk off. After rinsing the tank, I filled it up with water and added a cup or two of bleach (in a 29 gallon) and left it overnight again.

The next day I rinsed it out 3 or 4 times to remove the bleach and then was ready to set it all up.

you might want to put a double or even triple dose of dechlorinator in the water when you fill it to get rid of the bleach, which is really concentrated chlorine.

when the previous inhabitants of my 20G died from a mysterious disease, i took no chances and washed everything with bleach water, including the HOB filter, box filter, substrate, heater and ornaments. i got new carbon filter pads and new plants. after letting it soak for 2 days, i washed it and soaked it repeatedly for another 2 or 3 days. then i let everything dry out for another 2 or 3 days (mostly substrate that took longest). after everything was back in the tank along with new tap water, i gave it a larger dose of water conditioner (Prime) and that removed just about all of the chemicals.