I use canada dry "tonic water" never had a problem with a sticky film building up.... may be the brand you are using, check the ingredients throuroughly. Turtle, you need to think about transplanting very soon... I don't know why I didn't say anything before as this could be a huge problem... The most logical action would be to kill off the runts and transplant your two bigger plants into 3 gallon pots. Their roots are fighting for space.. this is gonna inhibit growth majorly.

Peace