I went to try and take a better pic of him, noticed the red has banded down further into his tail, it's spreading, and it seems that my neons have the same "rust" at the base of their fins.
If this is a bacteria I don't have fish antibiotics, I have opthalmic antibiotics for cats/horses.
Have you tested your water for ammonia, nitrites, nitrates & Ph? I'd start with that, hopefully you have a liquid test kit. If you don't have the proper test kit, do a 50% water change & then go buy the test kit. How long has your tank been running? Was it properly cycled before you added the fish? If not you are currently doing a fish IN cycle & it will be VERY important for you to do frequent water changes & would be even more helpful if you had a liquid test kit to check your water parameters.
Are you using the liquid test kit? You have 0 nitrAte & .25 nitrItes? Or is that backwards? If you do have .25 nitrItes then you need to do a water change because nitrItes are dangerous to the fish. When was your last water changed? Because all of your fish seem to have the same signs, I'm leaning towards nitrIte poisoning & I would do a 50% water change to get that nitrIte down to 0. I understand that .25 isn't too bad but with what you're describing I would still do a water change. Adding media from another tank won't cycle the tank unless you also provide ammonia, the BB will die off without an ammonia source, did you add ammonia while the tank was fish LESS?
Okay, so I ended up changing 25% water. My fish don't seem lethargic at all, they're swimming around top and bottom, eating, playing, sleeping etc.
If the nitrites still test later on I'll do another change. Oh and I have a liquid test kit.