java ferns killing my bettas?

cant be stress hes been in there for weeks before this started up. not contaminated food hes gone through 3 different cans of flake and has been eating bloodworms to. all my other fish are fed the same food and havent had this problem
 
How's the water temperature?
 
How long before the first betta died/started with symptoms did the Java fern get introduced? Did you clean out the tank and all equipment before introducing the second betta? What did you use?

The only thing that I can think of is 1. betta 1 died with some sort of disease and passed it to betta 2 when moved into the tank. OR 2. there is an equipment malfunction that is causing the temp or something to either vary too much or just stay at a suboptimal temp.

I would be really, really, really surprised if the Java fern is killing the bettas. Unless, it came from a diseased tank and that's why the bettas are dying.
 
temp is a stable 75. i added the betta about a 1 week or so after i got him to finish my fishless cycle in that tank off. plants were there from day1. this guy oddly enough is actually looking better today. i did a water change yesterday and he is looking more active today and fins havent started shriveling yet so he may be alright :)
 
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