Here's the short story. In the past year, I've had two bettas. The first one died back in March. His symptoms towards the end included lethargy, not eating, inability to maintain a depth (would sink to the bottom), and a reddish tint around the gills. My most recent one, Blue, died this morning. He was showing similar symptoms. He starting eating less about 2 weeks ago, and went from dancing around the front of the tank when we walked up to just staring at us. Then, Thursday, he would swim around, but his back half would drop down and he would look like he was swimming vertically. Then yesterday, he would just sit around, and showed no interest in coming to the top.
My tank setup is an Eclipse 3. I get my water from the local pet store, which sells pre-treated water for cheap. I have a heater in there which keeps it at 78. Originally, I didn't have any other fish in there for awhile, but just put in two otos and a small piece of drift wood Monday to help take care of some brownish red algae that was growing all over the place. They seem to be doing OK right now. I had been changing 25% of the water once a month when I changed the filter, as that's what the directions said. I realize now I should have been doing that more, but with the filter, I wouldn't think that alone would kill him.
What might I be doing wrong? I hope to get another betta to put in there sometime soon, and want this one to live more than 6 months.