Happily, there ARE things you can do to help nudge things in the right direction.
When you're in a chain store and there's a knowledgable GOOD clerk working the fish area---acknowledge them and express your appreciation for their good work even if you're not buying anything yourself. Give 'em some good strokes. If you've got the time, find the manager and express your appreciation to them as well, or send an email to the store. Make sure that employee feels valued and VALIDATED in their position.
If you're shopping in a store and have the opportunity to interact with folks who are looking at fish--DO. If they're experienced, great-you just had a nice interactin. If they're new/confused-take the opportunity to point out a few facts or comment on the fish they're looking at. If they're receptive, you can answer questions. If not, oh well--you tried. As for not being "that guy"? Communication is a skill, the more you practice the better you get.
Don't buy from bad stores. If it's an LFS, tell the manager why you aren't shopping there (dyed fish, misinformation, etc). If it's a chain, write to the corporate address and identify the store and your specific complaints in a polite and concise manner. Good feedback goes to the local level, complaints go to the next level up.
Don't buy problematic fish from stores that aren't selling them responsibly. "rescuing" fish from a store by buying them only encourages continued stocking of that item.