NEW FILTER? HALP!!!!!!!!!!!!!!!!!!?

so i have a fifteen gallon tank in need of a internal filter. i would LOVE to have a mechanical/chemical/biological filter and have heard the cascade 300 and fluval U2 are good filters. In my tank i currently have two small goldfish, but am thinking about getting another one so i probably need a good filter because goldfish are dirty. Does anyone know of any good internal filters for a fifteen gallon that are small, strong, and relatively cheap?