Depending on where you live, with your parents you can get on a local freecycle list. I am on one and occasionally all sorts of aquariums and equipment show up for free. You can also put out a wanted request for a larger aquarium. I wouldnt think that the maintanence on a 2.5 gal would be much more than a 1 gallon, and usually parents cant argue with free. The only hitch is that you usually have to be able to pick up the free item.
just google freecycyle and see whats in your area. craigs list also has a free area. but you would want to have your parents involved before contacting anyone or putting a wanted request out there.
didnt really check if this got answered or not yet...but i just thought i would say that i got my 5 gallon tank w/ filter, overhead lamp and cover, and heater from walmart for 30 bucks...so a 2.5 gallon should be cheaper than that.
Actually probably not. In general there is some significant up front costs in starting a fish tank, but the difference between gallonages is not linear. For instance I just priced out the cost of starting a 10 gallon tank, without decorations or fish, to be between $90 and $110 dollars. However going up to a 20 gallon tank will only add on another $20 dollars or so. Same for moving up to a 30 gallon tank, another $20 on top of the 20 gallon cost. The small kits can be inexpensive but it's not a linear reduction in cost.
Ironically when I compared tank prices at my local stores, 10 gallons were cheaper than both 2.5 and 5 gallon tanks! I thought it terribly strange, but hey, I'll take a 10 gallon, I've got the room for them, and I'm sure my fish don't mind the extra space.
Some hints for you on keeping your tank warmer if you still have not managed to convince your parents to at least let you have a heater: Make sure the tank is not near any place that is drafty -near a door is a good example, because every time a person opens it to go in and out, it allows cold drafts to come in, which causes your tank to fluctuate in temp, usually colder.
If you have a table lamp that goes on an end stand or something of the like, put the tank near it, it can help warm the tank something like a heat light does for a reptile tank. This is not very stable, but better than nothing.
In a general geographic reqion, where do you live? If you live far north, then try placing your betta near the southern wall of your house. This side of your home is exposed to the most sun, and therefore is actually slightly warmer. If you live in the south, then placing the tank closer to the east wall in spring and winter, and the west wall in summer and fall will have the same effect. In houses where there is not constant air circulation, these places will have a higher ambient air temperature due to the increased exposure to the sun, which will then affect the temp of the tank.
Consider placing your tank near a window in sunlight for several hours each day. This is risky, because it can sometimes cause extreme temperature flucuations, so you will need to monitor it closely, but hey, this is how bettas warm themselves up in the wild, the sun beats down on the water in the rice paddies all day. Bear in mind though, doing this in a tank (especially a small one!) has side effects! Algae is much more likely to grow in a tank when exposed to direct sunlight, consequently, you will need to change your betta's water more often. Also, the tank itself will act as something of a magnifying glass, and focus the sunlight and heat in the tank. Its something like what the glass windows of a car does in summer - you know how you get in, and fell like dieing because the inside of the car is so hot? It's like that. So as I said, you must monitor your betta's tank temp closesly, and not allow it to rise above about 82 degrees.
Do more frequent water changes. While the betta can get by only have water changes once a week (these are hardy fish after all, but hardy and happy are two different things), in only one gallon it is going to foul the water fairly quickly, and if you had a test kit (which I'm betting you don't, seeing as how your parents seem uninclined to let you have anything else) you would notice how the ammonia levels rise quickly in such a limited space. Cleaner water is never going to hurt the betta, and when you do a water change, take the temp of the new water you put in, and try to get it around 80-82 degrees (be careful though, don't get it too hot! 90 is like a sauna to bettas, don't fry your fish!). While this may be only temporary (well, all of my suggestions pretty much are) this will at least give the betta a little bit of time in warm water before it slowly cools back off to room temp.
Oh, and if you feel like being sly, being as you have no heater, your water temp being around 68 indicates this is also about the temperature you keep your house at. Try leaning on your parents about saving money by raising the temperature of the house a few degrees simply by turning up the thermostat a bit. This wil save them lots of money in the long run as AC bills can get quite costly you know *wink wink, nudge nudge* A warmer house in summer isn't going to kill you, and it would certainly help improve the betta's living condition (not that they need to know or care about that xD). The money bit I generally find works on the stubborn sort much better, push that line about how much they could save by not having their AC running as much.
Yeah, a heater would help. I got mine on sale for $18 at Drsfosterandsmith.com but I paid a $9.99 shipping fee, too. Ugh! But I really had no choice. Petsmart didn't have what I wanted and I have no transportation so I had to do it. I'm so happy for you that you got a 10 gallon tank! :) YAY! Only one of mine has a heater though.