4 Of My Fish Have Developped Dark Fins?

[wetdude92]

I have a 33g tank with an opaline gourami(3"), a red tail black shark(1.5"), 2 female and one male australian rainbows and 2 rosy barbs...
The rainbows are the newest addition to the tank, and they have developped thick, dark edges on their fins, and so has the gourami...the rest of the fish have not been affected as far as i can tell...

I have tried searching the net for any possible cause but can't find anything, and the guy at the fish store(quite knowledgeable) said not to worry about it...

Does anyone have any ideas or know what causes this? Should I be worried?

Thanks