I am building an animatronic alien inside a transparent plastic cylinder. The cylinder is filled with water and a few drops of green ink. The problem I have is: in a few days, the water fills with algae and becomes moldy. I have tried some mixtures (water plus white vinegar, water plus alcohol, water plus chlorine) and although the water last longer clean, the ink always reacts with the vinegar/alcohol/chlorine and the green color dissappears or becomes an awful brownish. Do you know any way to keep green water without fungi or algae?
It looks like you're making something rather like a specimen in a jar, so why not do it the way the professionals do it: pure ethanol or isopropyl alcohol?
As long as the jar is properly sealed, this should keep your alien fluid in pristine condition for centuries to come.
Of course, this may be overkill. If you do a decent job of sterilising the jar beforehand (I recommend the kind of steriliser used for brewing), boil the water beforehand and seal the jar properly, then even without additions the inside of the jar should stay bacteria/fungi/mould/algae free for months or years.