Help help pls help!

[Tails]

(ok this didnt seem like a fish disease - sorry to ask a mod to move this to where it should be)

My betta's fins are shredded! :'( His dorsal fin is all torn and his tail has been split :'( When I got him, he had a small hole in his dorsal fin which had only now just healed, and after a day or two in his new tank, the end of his tail fin was chewed (he was alone then, so it could only have been from himself).

Only two things in the tank have changed, I bought a bristlenose and a tank background. Now, in the background, he appears to be able to see his reflection and got all aggressive and puffy with it. I kinda hoped he'd get used to it, because I love the background and I've heard a few people actually keep a mirror in their betta's tank (which I would never do), and their betta's are ok.

Which do you think is the cause: the background or the bristlenose?

I'll be honest here, I'd rather remove the background than the bristlenose, but whatever the cause, I'll do whats best for my Littlebettabutta!

[Lady Hobbs]

How odd but I see nothing you mentioned that would contribute to this. Could he possibly have fin rot?

[sergo]

you didn't put the background in the tank did you? i know silly question.
probably fin rot. it seems that stuff will pop out of no where. how's your water quality?

[GoldLenny]

Did you quarantine the BN Pleco first? If not, it could have introduced some bacteria that your Betta did not have an immunity/resistance to.

Someone else asked but I'll presume you put the background on the outside of the tank. If it was a dark background, it could make the glass more reflective on the inside so the Betta is seeing himself and flaring. While this isn't terrible, it does stress them out a little until they realize it's their reflection. This added stress along with the possibility of a new bacteria from the BN could have caused the fin rot.

Many plecos do not do well with salt treatments so you should probably use Melafix. API also sells Bettafix which is just a weaker dosage of Melafix. I would just buy the Melafix and use 1/2 dose or 1/4 dose. I've used 1/2 dose on my labyrinth fish (gouramis) with no adverse effects. Since gouramis and bettas breathe at the surface using their labyrinth, the smell from the Melafix causes them problems which is why you should use a smaller dose than is recommended for "normal" fish.

Also do 2-3 25% PWC's (partial water changes), one every couple of hours, prior to starting the medicine and remove your carbon and chemical filtration from your filters. Make sure you vacuum your gravel good with future 25% PWC's since bad bacteria often will grow in the excess detritus in the gravel.

[gm72]

Agreed that you have an active infection. Plecos are not tolerant to salt and can be sensitive to medications. If you use Melafix I would recommend you use it at half dose or move your pleco to another tank while medicating.