What does the fish's poop look like?
How often do you feed the fish and what do you feed them?
What other fish are in the tank?
How often and how do you clean the filter?
What are the black things in the sand?
----------------------
Wipe the glass down inside and outside.
Do a 75% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Post clear pictures and video of the fish. Upload the video to YouTube, then copy & paste the link here.
----------------------
Feed the fish 3-5 times a day for 2-4 weeks, with a variety of frozen and live foods (don't use tubifex or black worms).
Do a 50-75% water change and gravel clean every day while you are feeding them more often so the water stays clean.
Check the fish's poop.
Test the water for ammonia, nitrite, nitrate and pH. Post the results in numbers here.
Maybe deworm the fish but post video of it swimming before doing that.