how do i tell the diffrence between water milfoil and parrots feather???

Parrot feather is a type of milfoil or Myriophyllum. Parrot feather is Myriophyllum aquaticum. There are many kinds of milfoil. This store sells four species and includes photos so you can see.

http://www.walliscreekwatergarden.com.au/smallplants.html

The main difference between common and Eurasian milfoil versus parrot feather is that the leaves of the plant are skinny and fine and not as fluffy (each piece) than on the parrot feather. Parrot feather also tends to be a lighter green.

There is now a red stem milfoil. Red stem milfoils and parrot feather are marginals but eurasian wtare milfoil is an oxygenator.