Originally posted by Ex Finn. But how to evict the ones that start to feel comfortable and set up shop, feeding on your precious lens-coatings
Only way I know is to keep the gear dry. That halts or prevents growth.
I have cleaned several lenses that I acquired with minor, and one not minor, cases of fungus. I found that disassembling and cleaning with rubbing alcohol worked rather well. Make sure to use pure alcohol, not anything that has additives like scents or moisturizers. Regular glass cleaner would work as well. But when the lens is open you have to think 'clean room', dust, finger prints, whatever that you might not notice when assembling are glaringly obvious once you think you are done.
The real hard part is taking the lens apart and putting it back right. Cleaning the fungus threads was not a problem. I have read of cases where the coatings were etched and thus the lens ruined but I have not experienced that. All the lenses I've cleaned came out perfect.
You can also get fungus in the glue between elements and in that case I think the lens is a lost cause. If it were extremely expensive it might be worth professional repair but getting lens elements back together correctly is beyond most amateurs and certainly beyond me.