Bite

[awesome_loser]

i have some 6 platies n 2 mollies, well i did, found today one of platies dead noticed yesturday that her back fin had what looked like a bite taken out of it then now i find her dead, any ideas what it is? cuz now i noticed 2 more of platies have what looks like bites taken out of their back fins dont want anymore to die

 

[mark.w.jones]

Could be finrot, are the fins looking visibly ragged and frayed? Usual cause is poor water quality but sometimes damage against tank ornaments or being attacked by another fish. Platies and mollies are not usually aggresive so it's most likely to be poor water quality.

Did you fishless cycle your tank following the add & wait method (link in sig.) before adding your fish?
If not then you are doing a fish cycle which is more time consuming and unfortunately, more likely to lead to death of fish.

Can you get water readings for ammonia, nitrite and nitrate for us? Either buy yourself a liquid test kit. Aquarium Pharmaceuticals Freshwater Master Test Kit is a good one or take a water sample to your LFS where they'll usually do the tests cheaply for you.