it's is possible to drill your glass,just take ur time in doing it,let the drill bit do the work and don't put alot of pressure on the bit or it will crack the glass.if u have a piece of glass u can try drilling that piece before u try to drill ur tank
I say go for it I drilled 3 holes in the back of my 75 and paid $26 for 3 bits.. Ok so they were from hong kong but, they got the job done! Take a peek at how it turned out..
I have drilled glass tanks before, please do your self a favor and have a professional do it; they will have all of the correct equipment such as diamond bit drill hole cutters.