The Phyto isn't just for corals, it's for the feather dusters, copepods, sponges, and other filter feeders that maybe/are present in your live rock.
I was told that LIVE phyto plankton also lowers nitrate levels as they themselves feed.
I have some live barnacles on my rock and some sponges that other people strongly recommended I remove because they would certainly die and cause ammonia spikes, but I fed rotifers and phytoplankton right from the start and have some pretty sponges because of it. (Having owned a boat, I can't imagine anything killing barnacles!
)
As for the amount, that depends on the size of the tank, I tend to OVERFEED everything, so I'll let others help you there.
I turn off the filter and skimmer, dump some phyto and let the powerheads push it around. If you don't skim
constantly, (and use
LIVE phyto) the phyto will tend to multiply, feeding off the nutrients in the system and increasing in numbers. I feed phtyo daily, I filter and skim overnight and often while at work, but I just run powerheads (AND feed) when I'm home.
Everyone runs different schedules as to when they feed, etc.If you ask, you'll get a thousand answers .
I'm sure different products have different types and concentrations of Phtyoplankton. I would try to use
live, if possible, so that they can help get (and keep) the parameters in balance.
Remember nature is always in balance, if there is too much of one thing, nature
will work to bring it back into specs. As long as the system doesn't crash while nature is trying to find the balance point, you're in luck!
Hope this helped,
Dosing Phytoplankton for corals? 
