Once upon a time, before the EU Tobacco Directive decided that tanks shouldn't hold more than 2ml, tanks holding 3 or 4ml were quite common, so one could happily vape all day without filling up (I always carry two mods and alternate between them - if nothing else, it vastly prolongs the life of the atomisers, as this method gives them a chance to cool down).
This simple poly tube increases the GT mini tank capacity from 2ml to 3.5ml. The poly tube for the wider GT tank increases the capacity of the GT tank to 4.2ml.
The only drawback is slightly aesthetic: the decorative metal grille outside the original glass on the Nautilus GT tanks can't be used together with this tube, as it's both wider and taller than the original glass. However, the poly tube is so sturdy that the grille isn't needed for stability. I have used my oldest poly tubes daily for more than two years and have dropped my devices on the floor several times, but the poly tubes are still as good as new.
Obviously, those who wrote the directive didn't realise that tank sizes can easily be altered with devices like this...

I already had the Nautilus GT tank and this find was exactly what was needed to make it a 24hr tank with no refilling. It’s superbly made, of course and does not suffer with any form of leakage. Thoroughly recommend this product.

The capacity of this tank is larger than I expected, it so good.
Also this tank looks so cool!