Superpet glass water bottle?

I have a glass water bottle for my guinea’s-not the most practical, but water bottle of choice- and i use bi-odor in their water. There is a ‘film’ in the water bottle, that i’m having severe trouble cleaning. The opening, as like most water bottles is tiny, so I cant get a scrub or wash cloth in there to clean it out. Some guinea’s drink from a water dish i know, and i tried that, but mine would rather swim in it than drink it, this is why i use a water bottle. There’s probably a simple answer to clean this thing, but im having a mental block in doing so. If anyone has any suggestions…it would be appriciated if you would share! Thanks!