I would put the angel in the 10g (it's a juvy right?). I have my juvy with the fin problem in a 10g and she/he's about the size of a quarter. I wouldn't want to treat a 55g w/any anti-bacterial unless absolutely neccessary (sp?), besides the cost you wouldn't want to mess up your biological filter.
For 10 US gallons put in half a cap full of melafix and half a cap full of pimafix for 7 days and do 25% water change on the seventh day. be sure to keep an air stone in there and don't have the temp to high. for one bacteria grows faster at higher temps, these meds (which are natural...tea tree oil) will reduce O2 levels, and if you do have an ammonia problem (I haven't yet on the 3rd day) I believe the higher temp makes it more toxic (could be the other way around but I'm pretty sure about this)
If I didn't have neons in w/my juvy right now I would do another 20% water change two days after that and then add a bit of aquarium salt.
good luck