First, I believe that water changes are an absolute necessity. I do a 10% water change on my tanks weekly. Which parameters do you match when you do the water changes? You are changing the same amount overall that I do but with ~20% change you create more shifts in the water parameters, including those you can't test for.
What are you running in the cannister filter?
Shedding the cuticle (slime coat) is a sign something is wrong. Part of the problem may be that you have 3 large fish in a relatively small tank which causes water quality issues. Your tank is also not yet mature and that also can cause swings in the water parameters. I am also wondering if the damsel isn't nipping at the fins? They are notorious and have been known to kill much larger fish.
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