no1-I am a glass merchant and have a glass company (not interested in building tanks)
No2- DO NOT USE 4mm GLASS
No3- Use the caculator
No4- dont skimp on thickness, go thicker if you can
No5- LowIon glass when viewing through will make next to no difference becasue water is also coloured
Glass thicknesses that are availiable here in NZ are 2,3,4,5,6,8,10,12,15 and 19mm glass. Do not use laminated glass as its only as strong as its thinnest sheet (3mm)