What are your thoughts between 36” and 48” deep?I had a 120 4x2x2 for a dozen years out of 1/2 and its still good. I upgraded to 3/4 just to reduce bowing on the 8 ft sides but I didn't want mixed sides so I made them all 3/4. No other reason they do a good job and would not build one too thin. I would recommend you go either 30 or 36 wide as there is almost no price difference and you get way more tank and much better rockwork options.
nice to have the extra depth but also worries about the bulkiness