As I understand it, the issue with taller and taller tanks is that the 'outward pressure' of the water against the acrylic tank sides near the very bottom increases linearly with the depth of the water. Thus the thickness of the acrylic sheet used has to match the outward pressure it will be subjected to. Small 12" tall tanks typically use 1/4" material (or slightly thinner). Medium sized 24" tall tanks typically use 1/2" material (or slightly thinner). Thus if you want to create a 48" deep tank you're probably talking about using at least 1" thick material, maybe even 1 1/4" to minimize any possibility of bowing.
To minimize risk of failure at the bottom joint, with 1" or 1 1/4" material for side walls and bottom you've got plenty of 'meat' available to use a router and make a 'tongue in groove joint' of say 1/2" by 1/2" where the tongue routed into the side wall sheets inserts into the groove routed near the outer edge of the bottom sheet (near would mean 1/2" from the edge for 1" material, or 3/4" from the edge with 1 1/4" material). Otherwise with a simple 'butt' joint you're totally depending on the structural strength of your acrylic glue to resist the 'outward pressure' of the water, which is not a fantastic idea with tanks this tall.
With a 50" length, you're also going to probably need two acrylic span braces evenly spaced across the top of the tank, or if you'd really like 2 * 24" of open tank top surface you could go with a 2"x2" aluminum angle top frame with a single aluminum span brace halfway across the top frame. Better still, find yourself a ready-made top frame with two 24" light fixtures meant for a 48" by 18" standard depth tank, and then tweak the length and width dimensions of your super-deep custom tank to fit this top frame exactly. With a tank as deep as this it's really important to give the tops of the sidewall sheets as much rigid support as possible, along with supporting the bottoms of the sidewall sheets, to help the entire tank assembly resist bowing.
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