Bacterial infection

[Buddacuda]

Hi, I'm hoping someone can help me with this one. I have a 150 gal with 2 threadfin lookdowns, a puffer, tessallated eel and horseshoe crab, some emerald crabs, serpent star, chocolate chip star. Anyway, one of the lookdowns has a internal bacteria infection. red areas on rt side near fin and Popeye on eye from secondary infection. No others show any symptoms. My issue is that if I treat gram negative bacteria with maracyn2, the invertebrates will die. I have nowhere else to put any of them. Also kill all good bacteria? I have fx5 filter, 2 reef octopus bh300 skimmers and change water regularly. Any suggestions? Also no lfs for saltwater fish. Thanks guys