The primary difference (for our purposes) between cell cast and extruded is their relative capabilities to handle stress. For pressure vessels such as aquaria, you want a material that can handle stress well. Extruded material is pre-stressed due to the mfg process itself and further stresses *can* lead to material failure. This is the primary reason to use cell cast acrylic over extruded for aquaria.
To answer the original question, I'd need to know if you plan on using any top perimeter bracing, AKA eurobracing and what the maximum water level will be when power goes out. 1/4" will hold water if you use a perimeter flange but is doubtful if you don't esp if there are any "less than perfect" joints. IMHO 3/8" is the thinnest that should be used and if no perimeter bracing - 1/2" (min) would be recommended. Also, please use a good quality material such as Cyro Acrylite GP, Polycast, or Plexi-Glas G (all cell cast acrylics BTW) .
HTH,
James