Congratulations, the female is probably still in the birthing process.
I use a pin to feed powdered food. I dip about 5mm into the water to wet the pin (you will need to adjust this as the population grows), then dip the pin into the powder (quickly so as to not get too much), then swish the pin in a big circle in the tank - as you have a large tank I would do this twice, once either end of the tank. As long as you don't wet too much of the pin I would do this every 2 days or so! Newborn shrimp tend to eat biofilm mainly, so I also use the bacter AE you already have, once a week (twice when there are an abundance of shrimp down the line)! The bacter AE I do as the powdered food method above, except use an old shrimp lolly stick (old matchstick would do) wet about 1cm and do that 3 times in different areas of the tank!
Simon