How do you clean a tank that's been just sitting?

Vinegar and a sponge. Don't use any harsh chemicals that may leave a residue or may be harmful to the tanks materials or fish.

You can use bleach but only use a teaspoon per gallon to disinfect rinse thoroughly wipe dry let it sit at least 48 for any bleach residue to dissipate

Call Nahackys...

I myself use salt (non-iodized) with water and a magnetic glass scrapper. Rinse well. Works like a charm for me.

I use non iodized salt and water..the salt will scrub about anything off then just rinse really well.

Salt and paper towel water