Depending what species you really have your heart set on and what your ambient temp is, you may need a heat lamp. I use one with my greys even though the house is around 68 degrees F because they eat better and are more active when they are warmed a bit.

I would start looking on craigslist and just get something slightly bigger. I know that I've seen tanks on there that are pretty cheap. You could even sell the 10g that you have on there and put the money towards something bigger.