If you're after a light tank and have money to burn, I can recommend the Fuel Supra Lite tanks. Was lucky enough to pick one up second hand and it's ludicrously light compared to most 'normal' fibre wrap tanks of an equivalent size. Stuck it on the kitchen scales along with my other tanks:
Fuel Supra Lite 68cu in with Ninja Pro Reg, filled to 2000psi: weight 976g / 2lb 2oz
DXS 68cu in with Ninja UL Reg, filled to 2000psi: weight 1386g / 3lb 0oz
SCI 68cu in with CP reg, filled to 2000psi: weight 1454g 3lb 3oz