
I take a ragular weight hammer and a 1/2" wide chissel and place the chissel near the edge of the edge of the resin build up and start taking a couple of wacks at it and then slowly start to work my way around and then slowly in towards the middle of the tank.
I did this a few weeks ago in my 2.5 gal tank as it was getting to be about 3/4" thick and i was starting to have trouble mking my 2 taller molds fit.
once you have it pretty much all cleaned out (or as clean as you can get it and are happy with it) give the inside of the tank a coat of mold release like you use in prepping you molds prior to casting.
i was thinking of using a hammer and large screw driver (as i used before), but i have the chissel sitting there from other work and thought i'd try it and it worked great!