Help plz
We have 40 gallon freshwater tank this morning I ended up finding 3 dead fish and I don't know why. Before we had ammonia spikes and that explains stuff but today I checked the water and it's was normal levels. We're new at this hobby and try our best to get everything right idk what happened here. In the second picture the Cory looks like it been nipped on or something I'm not sure. We use to have 3 Bettas 2 females and one male and we watch them make sure there were no nips or aggressive behaviors we've been good for months we've added a lot of plants for hiding spots and 2 big pieces of driftwood and they were getting along fine until last weekend our male betta looked like he got jumped and basically all of his fins got nipped and he was still alive when we found him and we put him in a 5 gallon tank to try to heal but he didn't survive then a few days later we found of our other betta female dead as well and she had eggs and it's just looked like she got nipped up too and her belly looked like it got eaten out we figured it was our koi betta since she was the biggest but as you can see in the picture she's passed from something and we can't tell what her eyes were looking a little cloudy yesterday night and she didn't wanna eat bloodworms and then today this morning she's dead . Any advice please would help us greatly we don't want to killing our fish.
EDIT: would like to include the fishes I have in this tank we have a 6 cories 1 otocinclus and 2 snails one nerite snail and one rabbit snail. I also texted my fish guy and he callede and let me know that based on the pictures (I sent him the same pictures I posted) that they gotten beaten up on and that it could've been the oto I would like to know if maybe anyone else would share that opinion. Reasoning that the oto just gets bored and starts killing 😔 I would hope that's not the case.