You basically either want to do the standard plate/float glass or a low-iron glass (like Starphire glass). The low-iron is slightly clearer and more expensive, but some people say it scratches more easily and some people think some kinds may yellow with age (I have not seen any empirical evidence to back either of these cons, these are just what I’ve seen people say). Standard plate/float glass (called annealed glass) is slightly less clear (many people note that they can’t tell the difference unless they see the two side by side), but it is cheaper, known to be the most scratch resistant material used in the hobby, and it for sure won’t yellow over time.
If crystal clear viewing is your top priority, and you’re not too concerned about scratching - go with the low-iron. If not going over budget (or not going over too far) is your priority - go with the standard annealed glass.
You do not want to use heat-strengthened or tempered glass (they’re both stronger than normal glass, but they both are less clear than normal glass too, and the way tempered glass breaks makes it considered a bit of a safety hazard for anything larger than nano aquaria).
Also, with regards to tank material thickness, most recommendations I’ve seen say to use the same thickness for either glass or acrylic, because while acrylic is stronger, it also bends more - so the extra thickness for the acrylic is basically to keep it from bowing out of proportion and busting a seam that way.
A couple random notes here:
- If you were to go with acrylic for the tank, you’d want to use cell cast acrylic.
- Here’s a simple but useful glass thickness calculator for aquariums (it’s generally recommended if this is one of your first DIY tank attempts that you exceed a safety factor of 3.8, so I’d say to probably set 4.0-4.5 at a minimum as your safety factor in the input box - the calculator may recommend a thicker, more expensive glass as a result, but the thicker glass, the safer it is):
Given below is the online aquarium glass thickness calculator which helps you to calculate a volume of an aquarium, glass area, glass thickness, weight of glass and weight of water in a glass based on the length, width, height, and safety factor.
www.easycalculation.com
* The glass thickness calculator linked above (like every other one I’ve seen) gives the value for an unbraced, rimless tank - bracing raises the safety factor.
** Safety factor is basically just a number to describe how likely a tank is to fail under normal conditions (i.e. assuming the tank is properly built, on a level stand, etc.) - the higher the number, the safer it is.