The growth of algae is caused by sunlight.  For my ferrets small (150ml)
bottles I use a toilet roll tube cut lengthways and placed round the tube
to shield them from direct light.It doesn't need to go completley round
the bottle, just shield most of the light Since doing this I don't get
any algae growing.
 
You need to remove the existing algae before you first do this .
 
Tony
[Posted in FML issue 5311]