I use 100% water from the main tank when I fill a fry tank, when I do water
changes I just take about half the fry tank water and tip it back to the
main tank then refill the fry tank with water from the main tank. I dont
filter my fry tanks for fry that small at all, just an airstone to keep the
water moving, once they are bigger they get put into a growout tank that is
cycled. I dont lose any fry doing it this way.