Bacterial Infection? (weird coloring on fin)

[RedJourney143]

Hello, I recently have been having a nitrate problem and I told my mother that mangroves could help. She took it upon herself to pick up some seeds from our local lagoon and put them in my reef tank without washing them. I was a little angry, but decided that if anything bad was going to happen, the harm was already done. So, I left them in. A couple days later my Royal Gramma does not show up for feeding time. The next morning I find the Gramma dead with weird discoloration on the rear fin. Now I have noticed slight discoloration on my firefish in the exact same spot. Is it possible this bacteria came from the mangroves, and if so should I take them out? Or was it the high nitrates that caused whatever happened to happen? And what should I medicate the water with?

Edit: Also the zoo where I volunteer has a similar problem with their fish but on the top fin, and I often put my hands in the tank that they live in, but I always wash my hands before putting them in my own tank.