Hello Bandardune and Welcome to Fish Lore. Sorry to hear that your fish isn't feeling up to par. Great suggestions above about the nitrogen cycle. It's crucial to fish keeping.
I think the best thing to do at this point is to do a 50%
water change and add some Prime or Amquel + to detox ammonia that may be in your tank. A water change is always a good place to start.
The Aquarium Nitrogen Cycle
Do you have a
test kit? What is most recommended is the
API (liquid) Master Test Kit. It contains all of the chemicals necessary to test for everything mentioned in the nitrogen cycle. Note: Paper test strips can be very unreliable and inaccurate.
Once you're able to test for ammonia,
nitrite and
nitrate and can post your readings, it will be a great help in determining what is wrong with your fish. Your readings should be: 0 ammonia, 0 nitrites and 5 to 20 nitrates.
Best of luck
Ken

I have moved your thread to the "Pleco" section of the forum for more respones.
Ken