I just finished doing our regular routine for water changings, i have 3 male bettas in a 20 gal, i remove them all with their little cups, only with one do i have to use the net to get him in there, the others calmly get into the cup with little effort. They are in the cups under a lamp for about an hour or so while i am empting water and sucking out poop. I keep an eye on them that they dont jump out but thats about it as i also have two toddlers running around.

So when i put them back in their tank this evening, one has his tail fin almost gone. I dont know how this happened he was never in contact with the other guys and he is not the one i use the net on.
Please help, there was no pieces of tail in his tank when i cleaned it and none in the cup either, they dont have any sharp plants or houses (((as i had one get stuck in a sharp plant and drown itself, lesson learned))). So i have no idea what could have happened to his tail, i've very sad and dissappointed in myself, i try to do everything right and now his tail is gone.

Also, i was overfeeding i found out so had 2 of them bloated, and now i am losing 1 due to the bloat or constipation or sbd, i tried fasting for couple days and the 1 got better completely, but the other still is not doing good. After fasting i did the pea thing, and he didn't really eat any of it, i let him fast for 1 more day and tried the daphina today, but so far no poop and he has lost his color and is looking completely sad.
So if anyone can please help, i am at a loss for this missing tail...