draing a old tank has lots of downsides, do first thing in AM.

drain valve can clog and not drain

drain valve can leak after draining. most drain valves are cheap
plastic, designed for one use to drain tank at end of life.

buy a drain cover, it screws on in case you have a leak.

be prepared to relace valve or tank, a friends drain valve was stuck
the handle snapped off, he tried unscrewing the valve, it broke. he
ended up replacing the tank..