You should complain to management. Tell them if they continue to keep the fish in such conditions you are going to report the store to corporate. Also tell them you will not continue to shop there as long as they house bettas in unacceptable conditions. If they do nothing find out the store number, look up the website for the corporate office and report the store. Tell them if the conditions do not improve you will contact the HSUS and PETA as well as alert the media.
Believe me, people that deal in selling animals want nothing to do with PETA. Thats a great threat to have under your belt when working in animal advocacy.
*Be smart.. Do your research!!*