This is my first post because I finally decided to actively seek some outside help for a problem my girlfriend's betta fish is having.
We got Gulpy, a male betta fish, maybe a month ago. We put him in a glass tank a little larger than a gallon (the aquarium was a Christmas gift and had no measurements on the box and there were no instructions), some pebbles, and a fake plant. The plant and pebbles were all washed in tap water thoroughly before they were placed in the aquarium and the tank itself was washed out before it was used.
We have been using distilled water bought in plastic one-gallon containers from the supermarket to fill his tank. Because of this we didn't not think we needed the water conditioner we bought just in case. We also feed him little pellets that are specially for betta fish.
Since we got him we have never seen him eat, he hardly moves, his fins have started to turn black (from being a pale blue) and he has started losing his color. Today I saw him tucked in a corner, his gills being the only things moving. We've changed out his water several times and washed the pebbles and plant, but to no avail.
He's obviously sickly and I'm afraid he's going to die soon. A few times I have felt his water and it seems cold. When I add some hot water he swims around a little bit, but no grand, long-term changes in his behavior have come about.
Please, what can we do to save him? This is our first time raising a betta fish and I feel horrible for making such a pretty creature so miserable.