I have two 2.5's and in one I have a 25 watt heater, this one:
And in the other I have a 25 watt heater from another brand,..I can't find it online, I'll check the brand when I get home.
But I don't recomend this brand at all, I had to return two already, maybe three cause the one I have in my fishes Romad's tank is leaving a cold spot where his plants are.
I think it's the Top Fin one, I'm not sure.
As FearlessLiter said it might be worth spending the 25$ for a good heater that u won't have to worry about.
I totally recomend the Hydor Theo works amazingly!!!