help indentify green 'bump' on fish belly

Crono

Black Skirt Tetra
MFK Member
Jun 15, 2022
49
10
13
Have you tested your water?
No
If I did not test my water...
...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.
Do you do water changes?
Yes
What percentage of water do you change?
51-60%
How frequently do you change your water?
Every two weeks
If I do not change my water...
...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.
how it started
buy new fish, quarantine. day later, white spot, add salt & meth blue.
few days later, white spot gone. to make sure, few day with only changing water, no medicine.

after the quarantine, grab the water from the main tank and add it to the quarantine tank. (yesterday perform)
in few hours, I spot fish spinning and 'sliding sideway', Internal parasite!?
change the quarantine tank water while adding medicine.

morning wake up, fish is dead (today happen). I spot green stuff under its belly, yet unable to find what it is.

the 2nd time dies. become sick after doing fine when in contact with the main tank.

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