is it on the inside or outside? also, is it new to you but previously used, or brand new and unused? did the residue appear recently or had it been there since you got it?
just to take a guess, it looks like lime scale typical in an older used tank.if so, it can be removed with vinegar and a lot of elbow grease (scrub hard with a plastic dish scrubber). if its on the inside you would need to drain the tank to clean it, and rinse well when done.
sometimes the glass gets etched, and if that's the case there is nothing you can do (reasonably) - I'm not sure what process causes it, but i have a few older tanks that even after scrubbing with concentrated HCL the haze never lessens.
alt="">
