Definitely not gourami, they are basically like bettas with lots of eggs and tiny fry. I've raised zebra danios pretty easily with micro worms and a 10gal tank. I would think some of you tetras you already have are spawning in your tank (if the conditions are right).
You have a 55gal yes? Perhaps look into bristle nose plecos. The male will take care of the fry and once they are free swimming they happily eat blanched green beans, algae wafers, etc. Be sure to provide some driftwood and several "breeding caves". Also, they often have several batches and many of their fry survive so you may easily be over run.
The only other egg layer that is easy is African cichlids, which I do not recommend as you have soft water, plus they are pretty mean.