So a few people have been asking about different colored glass for this tank that would fit. I have been trying to find some but nothing that is an exact fit.
If someone has their Merlin tank disassembled They could give this a try and see how much up and down movement there is when screwing the top fill piece into the chimney. If it allows you to screw it in or out further than the stock glass tank then these might work.
According to the specs from 3Fvape, The specs for the stock Merlin glass are as follows.
Diameter: 23mm (0.91 inch)
Height: 28mm (1.1 inch)
Now finding a tank with a 23mm diameter isn't that hard. The tricky part is trying to find one that is exactly 28mm tall. If the top piece and the chimney allow you to screw it in +- 2mm then some other tanks should fit.
I have found some tanks that are close to the height specs of the stock Merlin glass.
Replacement Translucent Red Glass Tank for SMOK TFV4 Tank (All colors on page are same size)
Height: 26mm (2mm shorter than stock Merlin glass)
Authentic Vapesoon Transparent Glass Tube for Ehpro Billow V3 RTA
Height: 29mm (1mm taller than stock Merlin glass If you wanted the extra capacity)
Billow V3 Styled RTA Rebuildable Tank Atomizer Red
Height: 27mm (Includes the whole RTA but just search replacement for this RTA)
I will let you know if I find any tanks that are an exact fit. If the top piece can adjust a couple mm either direction then these might work. You would just have to measure it the next time you go to rebuild the tank.