It sounds like there is a definitive possibility that you have some sort of crab or predator in there...
Most likely small, as if it were large enough both damaged fish would be gone.
A parasite could certainly be the case as well though...
If you post some pics, some of the hobbyists on this site could assuredly help you to figure out which is the problem in your tank.
then at least you will know what, if anything can be done...
You can use virtually anything as a qt btw.
Something as simple as a bucket, heater and airstone would suffice.
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