fish eat much more than just the "eyeful" that most experts speak of...fish do 3 things all their lives: mate, scavenge for food and play...food is a biggie in their lives
if the platies are constipated you can drop a few peas in (mush the pulp inside the shell out - discard the pea shell and just drop the pulp into the tank, once or twice a week will rid the tank of any constipation plus it is naturally nutritious
for 5 fish which you have, the fish should be fed as much as they are willing to eat within 2 minutes - spread the flakes or what have you over the tank so all the fish can eat
the key to feeding is not to overfeed; to do this depends on how many fish you have in the tank, less fish less food or time to feed; more fish just the opposite...so when you see the flakes floating about the tank for more than 2 minutes you know you overfed...just need about 5 pellets to do the job for 2 corys.
let us know how it goes