I got a new fish with a new tank this week, but I didn't have a heater to put in his tank. The first night he was fine, swimming around and checking out his new home. The next morning I checked on him and he was just hiding under his ornament, not moving much. Every time I checked up on him throughout the day, he was just lying there, mostly near the surface on his plant. He was like that for a few days. I'm planning on getting a new heater for him, but right now, money's a little tight, even though I should have one before next week.
I have another tank that's the same size with a heater that has my female betta in it. I only got that heater last week, and even without the heater, she was really active. So I decided to take out her heater (totally hated to do that to her, though!) and put it in with my new male, which I did this morning.
I just checked on him, and he's swimming around all happy!!!!!
So now I can actually see him, instead of trying to find him in amongst the leaves of his plant!
And that's the difference a heater can make to a betta!