4 deaths at once? :/

[Ritingyou]

So I had 4 female bettas in a 10 gallon with a couple guppies.. They never faught, never nipped eachother and were perfectlly healthy. I had decided to give spawning one pair a try and after going through all that process I took her back into the 10 gallon to heal. The first day she was swimming, eating, and healing like a healthy fish. Then over night I find her dead with a wound on her side.

Instantly I thought maybe they started fighting randomly so I took them all out and put them in their own tank. The other three did not have any wounds and still appeared healthy but the next day all three of them died with the same wound on the side! None of my guppies have been effected with what ever is going on..

I went to petsmart to ask my friend there (who knows a lot about fish) but she wasnt there.. so I asked the manager and he said it was just a stroke of bad luck with internal parasites... but I dont know how I got them in my tank in the first place... Could anyone explain this to me before I do something to cause this to happen to any others....

I was going to get some dividers and set both my 10 gallons up for my bettas (since I only have 6 left) but I am to scared to even use that tank now....