Is it possible for a betta not to like warmer water?
I have new bettas I was able to get the proper heater for two of them but the store was out of stock for the third so I had to wait a four days before I could get to another location for the third. I put the fish w/o the heater in a warm room the tank temperature stayed at 76. During that time the fish was very active and interactive. Now that the tank is 80-81 degrees he is not very active and less interactive. He isn't sick he eats fine, his water chemistry is fine. He definitely was a different fish in cooler water. Is possible that he just likes the water a little cooler?
Some fish seem to prefer cooler water, others prefer warmer. (Just like people! LOL) If he was at 76F, it may just be what he's used to, so that higher temp is feeling a little 'warm' to him.
How quickly did you raise the temp?
Maybe just back it down SLOWLY to 78F, like Shellieca said, and see if it makes a difference. I've seen people post ranges of about 76-82F as being 'comfortable' for them, so 78F could be a good starting point to see what he prefers.
I've definitely had this happen. one of my fish hates being at 80F, and prefers 74-76F. Another can only seem to function at exactly 79F, while my last one can withstand anywhere from 78-84F (he was shipped from thailand, though). I think it really just depends on the fish.
Unfortunately the type I use isn't adjustable but I can vent the tank more and lower the temperature. I will try that and see how he does if he still seems sluggish I will just take it out on the next water change. In a short time I will be buying an adjustable one in 3-4 weeks when I do a divided tank.
I didn't have the money to get an adjustable heater (college life sucks), but I usually do a thing where I leave the light on her tank for about 3 hours and turn off the heater, and when I am in class in the morning, I keep the lights off and the heater on. At night I turn both lights and heater off, because I can't monitor it if I am sleeping. She seems to be doing fine this way, MUCH happier than her old 60ish degree temp (I didn't do research... :( :( :( )
Sunshine, that method is actually quite dangerous, and you've given the reason yourself: you can't monitor it if you are sleeping. Definitely turn the lights off to let the fish rest, but by turning the heater off you could be causing temperature fluctuations that are far more dangerous than a temp that is slightly too hot or cold.
Mine are not adjustable but they do stay in a set acceptable range.
What size is the tank you have Sunshine and what brand/watt of heater? You may have too high a watt for your tank or it could be defective.
If it's not performing right I'd go the store you bought from and buy another one. Take the one back you have in the new box. You'll have receipt now and can get money back and at least be even. If the new one still does not perform right I'd take that back and get a different brand. A product should do what it says which should not involve overheating your fish. I'd just be sure to clean it thoroughly and wipe down with bleach, rinse well and air dry in case the store is not scrupulously and returns it to stock.