If this is a bare tank with no fish in it, you can use pure ammonia. It is the same thing that you would use for a fishless cycle. I found this out by accident when I overdosed a 33g that i was starting up. it cleans the glass amazingly with no work. I tried it again on another tank and with some minor scrubbing I was able to make a tank that was in awful shape look new. Rinse it well and the trace ammonia that is left, is part of the tank chemistry anyways. I also found that it was good for cleaning the gravel, especially if you are worried that there might be some nasties in it, if you bought it all used.
