Where did these fish come from!?!?!?

I have a fish tank. I had 3 zebra thingy fish in it, after cleaning the tank thuroly and putting in new rocks I put the fish back in the tank ( I forgot to put the stress coat in) needless to say they died. So here I have a fish tank with no fish not even a filter or light on. Just a tank and water and rocks. About a week ago my wife noticed something moving in the tank (tiny fish looking) now today as I am sitting next to the tank I notice 5 teenie tiny fish swimming around to tank. They have blue looking eyes and are transparent. You can see their back bones. I know nobody bought these cause if you saw this tank it doesn't look like anything could survive in it lol. Someone please give me a real answer or have I just sped up evoloution of micro organisms.