A few things...
Diatom blooms are pretty normal for new tanks. I doesn't mean you're doing anything wrong - it just means your tank is going through growing pains. Adding a cleanup crew of astrea and/or trochus snails will help as they'll eat the stuff up. Otherwise, they'll burn out over time.
Once your diatoms die off, you might have issues is you keep feeding every 12 hours. And especially if you're using an automatic feeder. If you're using an auto feeder, I don't see how you can tell whether or not you're overfeeding. If you're standing there watching, then why the auto feeder anyway? Many folks feed once every two days, with no obvious ill effects.
Also... phytoplankton is a type of food. I think if you were adding it, you'd know it. Why are you using Prime? Are you using it to treat tap water for use in the tank before it goes in?