I'm sure that someone will start talking about theft here eventually if they haven't already (I didn't hit page two), but I do have a comment on just that sort of thing.
I know that at the walmart in my area that sells bettas, they have a small "betta care area" next to the cups (on the shelves, just a cleared off area between the fish and the setups for small tanks, plants, food, etc. for purchase) where they keep the food and the Betta Water they have already "store used" (UPC's marked out, bottles partially full) from where they, occasionally, change out or top off the fish water. I would think that if you were to use *those* items to change out the water, then you would not, technically, be stealing from the store, because the store had already marked these things as "store use" expressely for the care of the fish. Now, if you were to take these items and use them on your own fish, then yes, that would be stealing. But using the already marked "store use" products on the store's own inventory, then, I would think, would not technically be theft. The argument could be made that you have overstepped your boundaries as a customer (but definitely gone a good ways toward being a decent person and caring for those unable to care for themselves), but I wouldn't think that theft could actually be accused.
(I used to work in a retail pharmacy, we would routinely use things like paper towels, dish soap, hand soap, Kleenex's and hand sanitizer for either cleaning in the pharmacy, or for overall patient comfort. These things were marked on the UPC's, and a list was kept of what was used.)
That's just my two cents and observation. :)