If its 30 inches tall and 150 gallons I would say 12mm or 1/2 inch glass minimum, no matter if its steel frame and especially if its home made, A steel frame need to needs to be stainless or it will rust and expand cracking the glass eventually. Also becasue steel expands and contracts at a much slower rate then glass it must have about 1/4 inch thick caulking to allow for the lag in expansion of the glass before the steel catches up or the steel will crack the frame.
Unless I was thoroughly impressed with his work, the glass was tempered glass stamped in each corner and it included a ton of extras I still would be very cautious with a home made less then 1/2 inch thick regular glass 150 gallon tank.
