cutting a 30 inch tall tank to 24 inches tall.

[sphelps]

You would have to take it apart and have the glass professionally cut, it'll be too thick for the average DIY to cut a 6" strip 72" long.

Personally I don't think it would be worth the trouble and you'd be better off buying a new tank but that's just what I would do. A standard 180 gallon tank is 6x2x2 and showing up with cash to somewhere like BWA would probably score you a deal if there was a tank in stock ready to go.