Molly death, guppie clamped fins

[Bluee]

Hello
So first of all, my water parameters:
Nh4: <0,05
No3: 1
No2: 0,05
Ph: 7,5
(edit: not mollie death, i didn't mean to write death but reaaallyy sick molly)
It is a 40 gallon tank with 1 gourami, 15-20 guppies and 4 mollies +occasionall fry that survives
Now i found a relative young black molly ill, he often lies on the side and seems to Hasehas trouble swimming. He is now in a quarantin tank. The problem is just that I now saw that some of my guppies have clamped fins, some more some less. I am thinking that maybe it's because of a internal parasite since I saw some stringy poop from the now ill molly fry. Does anyone of you maybe know what that could mean??

 

[Bluee]

I just remembered when I fed them, is it possible that the clamped fins are because they havnt eaten enough? Since I once had one dead fish that was probably because overfeeding I am now reallllyyy carefull and I think I gave them too less. They would just take like one minute to finish all the food. But sometimes I so feed them twice a day. One or two days per week I normally don't feed them anything. I just remembered that because I fed them just now and noticed that some of them have now less clamped fins. But obviously that could just be overreacting because I am really worried about my fishies...