This is a very simple problem to fix.  Algae grows because there are
micro-nutrients in the water and there is adequate light for photosynthesis.
Just put duck tape around the bottle and block the light getting to the
water.  Leave yourself a small open strip so you can see the water level in
the bottle.  It does not hurt your ferret for there to be algae growing in
his/her water.
 
Amy and her 11 fur snakes
[Posted in FML issue 2309]