I've used visi-therm or tronic heaters - get the 25 watt. That'll do for your 2 1/2 gallon (it's what i use for the 3 gallon tanks that I have). Both brands have adjustable controls with desired temps marked and built in thermostats (so they come on and off as needed to keep temps where you want them). They're a bit more than what you'd pay for the ones without controls. Try Dr Foster Smith or Big Als Online. Or even Petco should sell these types of heaters.

Do you have any PetSmarts or PetCo's or other "big box" pet supply stores near you?? If not, try their on-line websites.

I didn't have any problem finding a small heater for a 2-gallon tank. Many manufacturers are making them now due to the proliferation of small acrylic tanks manufacturers are making now. Don't worry too much about wattage so much as submergible length.

Thanks for the info Sky. I wasn't sure if the wattage would be too much for a tank this size. Thank heavens for the Gardenweb!!
Breezyb,yes I have both stores in the area. I called Petsmart and they led me to believe that they had no heater for a small tank. I'll check out their website as you said and compare prices.

You can get a Hagen 25W mini-submersible heater from animal world network for 11 dollars. Cheapest place I know of, and the heater is fairly reliable. And submersible.

Here is a link that might be useful: Animal world network

Well - the 25 watt is large for the the size tanks we both have - but not in the sense that it will overheat the tank - more that it's using more energy that is necessary to heat the tank.

I'm not sure they make a heater any smaller the 25 watt. Its the smallest I've ever seen. The standard measurement I've seen used is 5 watts per gallon of water. Thats one of the problems with the small table top tanks, you have to chose small cool water fish like white clouds or a pair of paradise fish to stock it or put in an oversize heater. I dont think if you chose a good heater you will have a problem with the oversize heater but it will probably take up a lot of room.

That's very good to know Sky. I was afraid of the 25 watt getting the water to warm. Don't want to fry the poor fish.
Guess I'm off to the store,lol.
Chris,so far that's about right. I have yet to see a decent heater for these smaller tanks( Someone is going to make a killing in that untapped market). I did however purchase a Jr. Heater from Walmart that's said to be for a 2-5 gallon tank(7.5 watts). I am just a bit concerned due to the fact that it has no thermostat on it. I have a small tank in a room that tends to be a bit cooler than the rest of the house in the winter and I don't want the fish to be subjected to water so cool that the poor thing would freeze.
Guess I'll get a 25 watt and see how that does for me.
Thanks all.
Grice

The 25 will be fine for a 2.5 gallon. I have two and never have had any problems with them; the junior heaters, on the other hand, tend to be unreliable and break easily. As long as you keep an eye on the thermometer - which should be happening anyway - the Hagen will do a beautiful job.

Thanks Minaku

A 25 watt heater won't use any more or less wattage than a larger or smaller heater. If the heater can regulate the tank water temperature it will only stay on long enough to heat to the desired temperature. A 25 watt heater would take twice as long to achieve the desired temp as a 50 watt would so the end result is the same. A better quality heater may be more efficient though. If your room temperature is stable around 70 degrees you could skip the heater.

Thanks James. I had wondered about that myself. You've saved me time,trouble,and maybe gas(driving to the store looking for a decent heater).
Grice

70 is too cold for a betta. This is not an opinion, but a fact. Bettas do best in temperatures between 78 and 80 degrees.

A 25W heater is chump change compared to a standard 60W lightbulb. I'd invest in one. There is a big difference in behavior between a cold betta (68-70) and a pleasantly warm betta (80). I moved today and couldn't keep my fish warm during the 6 hour car trip, so their water temperature dropped to 70. I was acclimating them to being on stable ground again and after an hour they were still listless, pale, clamped, and fear-striped. I added the heater, and they perked right up.

Right now my two fish are in 1/2 gallon transport tanks with a 25W Hagen mini-submersible in each. The heaters are doing just fine and have not overheated or cooked my fish.

I still suggest getting a heater. It's one of the best investments for your fish.

Hello to all who sent replies.
I just wanted to add an update.
I purchased a 25 watt heater as a 'test' for one of my 2.5 gallon tanks. I got the 25 watt and it works great for my betta Bradley. It's a 8 inch(I think) and does'nt take up too much room and has kept the temp consistenly at 81 degrees so far. So now I am certain that come this winter(rats!I hate to even think about cold air and snow!LOL).
All that's left is to get two more for my other two tanks(after I upgrade them all to 5 gallons).
Thanks again for your comments.
Grice

Minaku, I've had a betta in my unheated 135gal indoor pond for 3 years. Room temp goes from 65-90 in the course of a year, mostly around 70 degrees. The fact is bettas are very adaptable to water conditios and temp.

Does your betta live with koi, James?

It's true, bettas are hardy fish and will be able to acclimate to pretty much any kind of environment. However, they thrive in water that's 78-80 degrees. And isn't our job as fishkeepers to keep our fish as happy and healthy and in the best environment possible?

If you have a small tank with one or two fish (2.5gal) you can find a heater for your tank at petsmart not adjustable because its only 7.5 watts I have one in my 2.5 gal tank with a miniature clown plecostamus and a miniature peacock ciclid both from the same African river. They have been together in this small tank for just about a year, the pleco a year longer. Petsmart is the only place I found to have all sizes of heaters without going online.