Why doesnt Charlotte turn into a mermaid in season 2 when she cries?

Emma starts crying when she break ups with Byron and she doesn't turn into mermaid either...

i just don't think they turn into mermaids when they cry. Coz cleo was crying, quite alot actually and she didnt turn into a fish. I mean mermaid

I guess it's because it is their tears. Moisture coming from their bodies like sweat and tears might not affect them. Imagine if bodily fluids did affect them, they would be transforming into mermaids when using the toilet, and if it's a hot day the sweat will trigger the change. It would cause the girls to hide away in their rooms all the time.