I ordered a super sized lamp years ago (the 2 foot globe one), it came cloudy so they sent me a new globe for it, it came cloudy too (shouldn't have ordered in warm weather).

Anyway I re-watered one of the globes and changed the color, using water, salt, and a little dish soap. It worked well after a lot of adjustment but after several years, it grew some bacterial looking film/scum which looks nasty. I opened it up again today and filtered out as much of the scum as I could (that wasn't lodged in the wax).

Do any of you DIY experts know something I could add to a salt/water lamp that would prevent bacterial growth without breaking down over the years, clouding the water, or radically changing the density balance?

I have access to some less common things like powdered EDTA and various fish antibiotics, just not sure what to use that will hold up over years.