It very well could be because they were in to small of a tank. I know diamondbacks like their swimming room and space. If you were going to get a new tank I would suggest atleast a 75 gallon but if you can get like a 90 or a 110 I would go with the biggest you can get. If you don't want to spend a ton on a new tank look on craigslist.com , you can find nice deals on big tanks and save you like a chunk on money. I would also suggest leaving them apart for a little while so your males feet can heal, if their are ever open wounds on turtles any other tank mates see flesh and want to eat it.

Hope this helps,
Mitch