Heat tipped [cis] darts allow popping of Frozen and Lead Bloons.
~ BTD6 description
Heat-Tipped Darts is the second upgrade of Path 2 for the Monkey Sub in BTD6. It allows the Monkey Sub to pop Frozen and Lead Bloons. This upgrade costs $255 on Easy, $300 on Medium, $325 on Hard and $360 on Impoppable.
This upgrade will give the Monkey Sub red paint near the front of the sub, underneath the gun, as shown in the artwork. Purchasing two of the Path 1 upgrades will give the sub a green cap and green-lensed binoculars. Purchasing two of the Path 2 upgrades will give the sub two guns, as well as yellow paint around the window on the side of the sub.