I'm going with mosquitofish (G. holbrooki more likely than affinis). I'm not seeing any indications of golden topminnow - wrong body shape, wrong positioning of the dorsal fin and anal fin, not a short and 'thick' caudal peduncle, no blue or gold flecks). Based on body shape, long and narrow caudal peduncle, (lack of) pigmentation and the above characteristics, I am willing to bet that this is a mosquitofish. Also, do you notice the three faint rows of spots on the caudal fin? Most larger Gambusia I see have this..