Significantly stronger by design, Highland's TOUGHTANK is engineered to withstand unexpected internal pressures and minimize the chance of rupture due to improper filling, faulty equipment, or a clogged vent. TOUGHTANKS are UL-80 labeled for in indoor or outdoor use and are available in 137, 275 and 330 gallon capacities. For added protection, the 275 and 300 gallon models are available in double-wall construction.