How long have you had your fish and the tank?
I've seen this happen to fish that are newly bought and put into a brand new uncycled tank. Hearty fish can be hearty, but they too have their limitations. But like everyone says above, get the fish out of that pebbled water.
I had the fish in this aquarium for nearly 7 months. It was just an inch when I bought it. I did a complete water change last Saturday and added the water conditioner. But yesterday night at this time it was playing very well. All of a sudden when I woke up today morning it was struggling to keep afloat and his left eye appeared cloudy.
As of now, I have done a 50% wc in the morning, followed by 25% and 25% wc. I have also added 10 ml of paracidol and half a bottle of metrogyl solution as per instructions from a local pet store. I hope he can somehow survive and be healthy again. How frequently do you all suggest me to do a wc in this current situation.