A light "film" on my brackish tank...

I did a water change for the two puffers in my 10-gallon last Sunday,
and today have noticed a slight film on top of the water. It's a
little hard to describe, but the water is almost imperceptibly cloudy
at the very surface -- I wouldn't notice the cloudiness were it not
for "clear patches" here and there on the surface.

This "film" is easy to break with my finger but re-forms quickly after
I drag a "trail" through it. The puffers seem to be in good health and
good spirits. I didn't do ANYTHING unusual to the tank when I did the
water change (about 50%).

I am putting in about 3/4 teaspoon of aquarium salt per gallon of
water when I do the change, eyeballing it (I change about 1/2 of a
10-gallon tank, so I put in about 4 teaspoons of salt over the course
of the water change).

The puffers are eating blood worms and occasionally bits of red
wriggler (kept for "voiding", well-rinsed and frozen, then thawed).
The filter on the tank seems clean. No weirdness otherwise.

Thoughts/suggestions??

- Matt
www.man-man.org