My 180g is 10mm, the deeper the tank the thicker the glass has to be. A 24" deep tank only needs to be 10mm thick.i always err on the side of caution when doing things custom. Id rather have someone to go to for warranty issues when buying then having to rebuild when homemade. 10mm is a cm thick glass. if its a 100 gallon tank....u may want to go thicker, just to be safe. It is custom, make it last