You can shim it as was suggested above. If you do this, make sure to support it well enough and evenly enough that your stand won't break. I'd be particularly worried about this if the stand isn't hardwood, or well-framed.
You can also just put something under each corner and/or side of the stand. For instand, I have my 4' stands leveled by placing two of their feet up on 3/4" blocks, then using playing cards to completely level each corner.
If it's your house, you can pour a new spot for the tank using self-leveling concrete. It's not too hard, but it's more permanent than just leveling your stand.
You could also just build a level platform to be set on the concrete. You'd have to cut wood to fit the slope of the concrete, then place a piece of plywood on top of the frame so that it's completely level. If you do this, make sure the weight bears down on the frame, not just the plywood.
Additionally, you could buy leveling legs for your stand, or even get the gliders that self-level. Just make sure either of these are rated for the kind of weight your tank will put on them.