For a tank of this size (depth) you require structural grade silicone. Generally this is mentioned in the products technical specification sheet and equates to >30 MPa tensile shear so it's not surprising the DAP failed. I had a quick look at ASI Aquarium Silicone which has about 3 MPa tensile. However since you have already used it, you might as well do a water test once it's cured.
The highest stress zone is midway of the longest panels at the bottom so an internal glass brace along the long panel at bottom greatly increases the adhesion area - similar to having a bottom panel twice as thick. Most big tanks run the bottom brace along all four sides for uniformity. Some very big tanks (8m +) use a split base to enable movement.
If you do have to tear down and reseal, use structural grade PU instead of structural grade silicone because you can simply patch any future leaks with additional PU as new PU will bond to cured PU making repairs or modifications very simple. The structural aspect of PU significantly superior as well.
However structural glazing silicone will suffice.
Tempered glass is far stronger than float glass but is more flexible. Good choice of glass but requires more brace.
