How can you tell which way the joists run if the room is finished underneath?
There is an assumtion that the joists are "horizontal" meaning "parallel" to the 6ft dimension of the tank.
If this not the case , it makes a big difference
In that case of parallel joist support and located on a outside wall......
The tank load is 2000lb/12ft= 166psf. If is located on a outside wall and the joists are parallel to the 6ft dimension such that 166psf/2= 80psf. It divides load in half because the wall is non-deflection uniform load bearing This is the calculation of the condition that the joist is at least 16in from the wall and taking half the load.
80psf is twice that of 40psf, but less than 100psf live load. It will hold under ideal conditions. If it is a 2x10 joist then thats the only thing carrying 80psf load and the floor will sag. Get it wet ..
Assume it will get wet; which is far less than an ideal condition.
If it is sturdi-floor, then the I-joist or truss are 32in o.c. possibly it will be sitting only on press-board flooring. Strong when dry, but wet..oh dear...
If it were, say, the house that I built, and it used reclaimed #1 spruce 2x12's on 16ft span at 12in o.c., crowned up, with real . 75 dim plywood , then it would hold an armoured tank even when wet.

yet, I still prop the floor
You need a engineer only to get a stamped blueprint, then he's liable. If you dont change anything and dont get a drawing, its just an opinion.
I'm with yuz guyzez on opinions.
Do not not try this at home doubled up by a wealth of misinformation.