I need help figuring out what is wrong with my mandarin

[IronsDan]

tl;dr : Mandarin has bad fins, don't know what is going on whether fins are being eaten or disintegrating due to bacteria, was fine for a while then it started, dosing pima and melafix, could be: pest in rocks, bristle worms, bacteria or fungus, or possibly scraping on crushed coral. pics below

 

He was doing awesome for the first month I've had him and all the sudden his fins took a dive and started to either get eaten away or disintegrate. He is in a smaller tank 12g but I have a large HOB refugium that adds about a gallon and filled with pods and he readily accepts both frozen brine and mysis shrimp which I feed every day alternating food type. He is the only fish in there and the invertebrates in the tank are only 2 snails as far as I can tell... However there has been some clicking noises that I thought might be a pest mantis shrimp or pistol shrimp after realizing that it was not in fact the mandarin eating pods off the rock, but it seems too quiet to be a mantis or pistol, it's more like snail shells hitting rock or glass. There is ~10-12 pounds of live rock in the tank but the mantis or pistol shrimp would have to be tiny to be in any of them and I was thinking that if it was either that it would have eaten the snails already. I've been dosing pimafix and melafix everyday to try and stop them from degrading in case it is bacterial or fungal but it only gets worse and the coral I have in the tank does not like it at all when I dose the meds.

 

Important other information: I do water changes of 10-15% twice a week (trying to keep the phosphates from his eating down) so I know the parameters are not the problem, it does appear to have a white line along the ends of the fins that is why I thought it was bacterial, the weirdest part is that ONLY the bottom fins are being affected... his pectoral and anal fins and barely a sliver of the bottom of his tail fin are the only affected fins not the fins of the sides they use for swimming or his dorsal fins (I hope I used all those fin types correctly), this made me think that it might be that my having a crushed coral bottom was the problem, like his swimming across it cuts them or something possibly?

 

He is very still at night when the lights are off all the way so I was also thinking maybe some of the bigger bristle worms (when I say big I mean like 4 in, so not huge ones) in a search for food found this still fish and were getting at the lower fins since the bristle worms come out of the crushed coral substrate and rock work.

 

I really love this fish and I really don't know what to do to help him or even what the problem is so if anyone could help me find out how to make him better  if that means getting him to someone with a bigger tank I'll definitely do that. Thank you!

 

Pics (I did the best I could he wasn't flaring for me) :

In this one you can see that the fin it eaten away and see a little bit of white fuzz on the end and that the see through blue fins are perfectly intact

 

pretty useless picture but you can some what see his full tail fin

 

again a not great picture but testament to how much he eats which you can see because he's a some what thick boy

[Humblefish]

Not sure of the cause, but being he eats frozen it might be worth a shot to QT and treat with Kanaplex. Kanamycin is a mild antibiotic which is readily absorbed by the skin/fins.

[IronsDan]

Thats what i was hoping people would say because i got a 10g that im turning into qt right and I'll look into kanaplex too thank you!