Assuming you're using inches, not centimeters:
1)With a rimless tank, the thicker the better, but you might be able to get away with 1/4" glass (I'd personally brace it if you did that though). The safety factor would be ~3.9 for that tank without bracing assuming your workmanship in building it is flawless. Not a bad safety factor, but it's generally recommended to aim higher if you don't have much or any tank building experience.
2)Both ways work, but they impact how the tank works with the stand (see my quote below) - long story short, it would probably be easier if you don't have experience with tank building to go with the panels on top of the bottom panel, but the choice is yours at the end of the day.
To my knowledge, all three are used with success, but the one you go with impacts how you should support the tank on the bottom.
From a super old (2009) thread on a different site (reefcentral.com) by the user uncleof6, who built tanks for a living:
“Neither way is right and neither way is wrong. There are two schools of thought on this. The difference is in how the tank is supported. Sides on the bottom, behaves as an acrylic tank. The bottom needs 100% support (plywood top, and front to back beams every two feet under the plywood), foam is used under this type tank, to alleviate point stress due to irregularities in the plywood (not level or twist issues). Raised bottom, (or bottom inside) behaves as a rimmed glass tank, requiring support around the outside perimeter. No foam is used on this style of tank, as it COULD cause the bottom to fail. (break). Honestly, for a first time builder, the sides on the bottom is easier from a construction standpoint.
As far as tempered glass goes, use the proper thickness of glass, and tempered glass is unnecessary. Manufacturers use tempered glass so they can use thinner glass, less weight, less shipping costs. There is not really any other advantage to it.
Regards,
Jim”
He addressed your styles 1 & 3, in that order, and recommended style 1 for a beginner tank builder. I’m not sure how you would want to support style 2, but I know it has been done before without issues (I believe it has fallen out of popularity it recent years though).
3)The recommendations that I've seen for the silicone are ~1/16" or 1/8" - some people use toothpicks as the spacers (the average toothpick is just slightly larger than 1/16"), others use silicone spacers.
4)I honestly don't know that it matters so long as you get them squared up and straight/level.
5)Having done some research on this myself, it seems that pretty well everyone picks one and sticks with it until it fails on them. So, that said, I don't know that one silicone sealant is really any better than another - as long as they're aquarium safe, they should be fine. The one's that I see recommended the most are ASi Clear Aquarium Silicone Sealant, GE Silicone 1, Momentive RTV100, and Aqueon Silicone Sealant. Basically, as long as they don't contain mold inhibitors (like GE Silicone 2) or other toxic ingredients, and as long as they aren't expired (I've heard some horror stories about people buying expired silicones), they should be fine. Supposedly, GE 1 and RTV100 may contain small doses of toxic material, but I can't say of heard of anyone having problems with them, so take that with a grain of salt. Regardless, personally, I would say to go with either Aqueon or ASi.
Hope this helps, and good luck with your build!
