1: Go into makers' perch and look up - you should see a rim around top of room.
2: Now, you need to get on that rim. This is possible by flying in and up with a bit of lag.
3: Run round, as near to the tank as you can get.
4: There is a sticky out decoration under you. Jump onto its middle 'lump', and you should see a bit of space under it.
5: Use a speed boosting skill (eg Skull of Impending Doom, potion...) to run through this - you are now in the tank, where you can breath and mess around!
NOTE: i have been unable to do this as a Night Elf, but could as human, so I guess the height is quite strict - race changers or Baby Spice suggested?