20% water change with ammonia 2ppm will remove only 20% ammonia so you are left with 1.6ppm ammonia in the tank which means your water change is a waste of time. You need to do as big water change as it's needed to remove all the ammonia(90% at once) and then keep the ammonia as close to 0 as possible and below 0.25 with additional water changes which could be even twice a day. Water changes are not done for the sake of it but for a reason. This is especially important if you have fry in it, they rarely survive if they've grown in tanks with toxins. If they don't die, they may get stunted, develop deformities, been sickly all their life which may end up to be just a few months.
Livebearer fry actually don't mind big water changes at all, it's a benefit for them and stimulates their growth. As long as the water is temperature matched and dechlorinated, there should be no problem.