I have the same size 40L quarantine tank as u, however I am running a Ehiem canister filter. A sponge filter like yours may not be good enough, so I suspect the following reasons:
- ammonia spike somewhere, though u maybe measuring 0ppm at certain timing
- Ph shock maybe when u change 30% water
- copper poison
- temperature
- already a sick fish, and u further stress it with the small tank and copper dosage, and instability water parameter
I suggest u install a canister filter to stabilize your water, should you need to change water, use your main tank water. That's what I did when I quarantine a 5" powder blue for few weeks. Do not overdose copper, if you are using cupramine, I am dozing 15 drops and observe fish behavior..