You can, as long as it's level and make sure you don't place the stand on the tack strips along the wall that held the carpet.
However, there are several reasons that I don't recommended put it on top of the carpet.
-Salt creeps or spill would ruin the carpet.
-Should unfortunate accident such as overflow or flooded basement, you can't remove the carpet without dismantle and move the tank.
**If you're handy, if not hire a carpet installer to do this**
If possible, I highly recommend pull out the carpet along the wall just enough for the dept of the tank. Place the stand on the padding (if there any) and cut out the padding along the tank stand (cut out about 1-1.5" along the stand). remove padding that you cut out. Place the stand and tank into the cut out area. Level the stand/tank. Install carpet strips (if concrete use "concrete carpet strip) along the stand leave a 1/4" gap from the stand to tuck the carpet in. Cut the carpet along your stand, leaving 1/4" extra carpet (so you can tuck the carpet in between the strips and the stand).