Our fish are dying. We checked our amonia, nitrate, and nitrite levels and they seem to be fine. They're not getting bullied, its the same group of fish we've had for about 6 months now. Right before they die, their fins look a little ragged. And within a few hours they are dead. Their bodies appear to have red marks on them. I'm guessing it's some sort of bacterial infection but i'm not sure what to medicate them with. They're dying fast! They usually die within a few hours of noticing that they are sick.

Sorry to hear of your problems. :(
I'm no expert on infections, cause all I've had to deal with is dropsy. But I seem to recall someone posting recently a website with a list of fish diseases. On it was a condition with symptoms being red sores...wish I could remember what it was.
I believe infections thrive at higher temperatures so perhaps slowly lowering the temperature of your tank/dosing with small amounts of aquarium salt might help. The salt will depend heavily on your fish stock..I'm not really knowlegable on gouramis or plecos, but the rest of your fish may benefit from small doses as a preventative after water changes, and especially now to treat whatever may be bothering them. Be sure to follow instructions carefully. I actually dose less than the package recommends because my tetras, kuhli's, and well, most of my stock are sensitive to salt. Also remember that salt dissolves into the water column and stays there, whatever you add won't be removed with evaporation, filtration, etc. Only with water changes.
Which fish is it exactly that are kicking the bucket on ya?
In the meantime goodluck.

Thanks for the recommendations. We have lost a Lyretail Molly, Sunset Platy, Gourami, Blue Ram (looks like the other is about to croak). We have started dosing with Erythromycin from API. Hopefully this takes care of it. We also added a dose of salt. From researching, we found that it could be Hemmoragic Septicemia (not positive). I don't think it is fungal, but bacteria. Hopefully the treatment doesn't harm our beneficial bacteria, but at the rate they are dropping, they are all going to be affected by tomorrow.

I was going to suggest heavy duty antibiotic. It's sad to hear they're going so fast. I've had fish in the past die of bacterial infections that seemed to paralyse the fish slowely from the tail toward the head. Tragic. I would say research salt as Spongbob said I think too.