All I have ever owned is Platies, and I recently went through the same thing that you did, all at once they were dying and I couldn't figure out to what I did wrong or what was wrong w/ my tank. I lost all of my females, all the fry, and all but 2 males.
I did water changes, checked the param's on a weekly basis. I took water to the LFS (very reputable might I add, not like the petco's or petsmarts) and they checked my water....everything fine. And still my platies were dying. I got so frustrated that I couldn't see strait, and when I came here I got some wonderful advice and even some of the members made me rethink some of the things that I was doing.
Now, when and where did you buy your platies at? The reason I ask is b/c if you did purchase them from the LFS (like a Petco or Petsmart) then they very well could have a weak immune system, and since your filter wasn't working correctly then that would cause them stress, which will lead to thier death. My platies were dying b/c of old age-plain and simple.
Of course, I am no expert in the regards to everything that there is to know, and there may be somebody out there that knows a million times more than me! I would keep doing the water changes, adding the declorinator, and watching them very closely. I hope that they pull through it!
Charity