I've had my betta, Elton, for about 4 months now. He had a couple cases of ich, but was treated and has been fine since then up until about a week ago.
I have a 2.5 gallon tank with filter and heater (and thermometer), and do appropriate water changes, water treatment (prime), etc. (according to aquarium betta people).
A week or so ago, I did a full water change (I do this occasionally bc I don't have aquarium plants). The temp was fine, and it was treated. He has always been fine with this. But, I forgot to plug in the heater this time :( and the temp dropped to room temp (around 72-74). I noticed the next morning and plugged the heater back in and it has been at normal temp since.
My fish has been very, very lethargic, and is paler than usual. He lays at the bottom of the tank almost all day and leans to his side. I've never seen him act this way (he was pretty lethargic when he had ich, but no ich spots this time).
The aquarium people said he is temperature stressed and getting over it, but it's been a week! Also, when I first got him, his tank was by the window on a particularly hot day and the temp rose up to around 85. I got home, noticed, and dropped it to a safer temp. He was frazzled for about an hr or so, but then was fine. No long-term stress from that (this was months ago).
The underside of his mouth/jaw is more silver and he's generally more pale. Nothing else is visibly wrong with him - no spots, no fungus, no growths, etc.
The other possibility is that my mom thought it was cute to feed him and he ate too much (she claims it was only after he started looking sick though). I had a very long talk with her about fish care. He doesn't really look bloated though, but I'm fasting him for a day to be sure.
Does anyone else have any ideas? Elton & I appreciate your help.