5 gallon tank cycling help ??? ?

I have a 5 gallon tank that I was bamboozled into purchasing at a local petshop. The shameless clerk sent me home with a 5 gallon tank with filter and heater, and 2 corycats. I know now that my corys would enjoy a bigger tank, but I'm attached to them, and I have no room (or funds) for a bigger tank. I'm about a month in, and my tank just isn't showing any signs of cycling. I though perhaps low oxygen conditions where to blame, so I bought an airstone and airpump. I'm dechlorinating the water and doing changes to keep my fish healthy twice a day. If you need any additional details, let me know, I'll post them! Thanks :)