Hi.. thanks for the replies. I came here everyday and without any reply I was at a loss and my fish's condition has become worse.
I have done something extremely stupid! It was in a smaller tank before and I transfered it to an empty marine tank. I filled the new tank with freshwater and checked the water for salinity and it was fine for a freshwater fish. My FH was fine in its new tank for 2 days and was eating properly.... and no adverse condition was noticed. Then I changed the FH to yet another tank which was a freshwater tank always. It was fine for a few days and suddenly one morning I found it standing upright in one corner and the body half way had turned black. I have a feeling the salinity in the marine tank, empty except for the gravel, had risen to unsuitable level though it didn't show while testing salinity again!
Right now it has become worse and lying on its side and the body has bent horizontally... opposite of bending from the spine. It's breathing heavy.
It's been more than a year with me... don't know the age, but it's around 10 inches in size.
I wanted to give it a bigger tank... but I think somehow the salinity became a big problem. I can't think of any other reason for it to become so sick all of a sudden. It had earlier faced a 4 hrs of power cut in the smaller tank when I had lost a few other fishes in a bigger tank. It seemed like a strong fish. I still don't know if the salinity is the problem... but I can't see anything else that I did wrong...
Any help would be great.
