Issue is more than just permeation
Walls also have to be thick enough to support the weight of the contents (8.33 lbs/gal)...otherwise the tank bulges, flexes and becomes susceptible to cracking. While 1/4" is thick enough to support up to about 20 gallons (unless the tank is exceptionally tall and skinny) the wall thickness should steadily increase just slightly as the tank size gets larger, to 1/2" or even thicker as tank size passes 60-75 gallons. Note though--there is no hard and fast rule for wall thickness vs capacity...it depends as much on the shape of the tank as the actual size. When it comes to permeation, wall thickness is a factor...but quality of materials and mfrg--especially the curing process--are more of a factor. A good quality 1/4" tank up to 20 gallons will last 15-20 years as long as it's not subjected to repeated stress from a blocked vent and/or fittings aren't over-tightenend. However, a tank from any mfr who starts from a minimum 3/8" wall for even the very smallest sizes will last for decades, and is far more able to withstand pressure or suction from a blocked vent. Bottom line: IMO, you cannot do better than a tank from Ronco Plastics.
