Very strange title I know. I had a planted tank for about 6 months now, its totally fine, with some algae here and there, otherwise perfect water parameters. I've had a shoal of 6 cardinal tetras since about the 2-month mark. I recently (3 days ago) added 3 guppies, two female and one male. The first night the male went missing, never found (at least not yet), second night one female was on the bottom with no fins. I have one female left and I had no explanation for why they had passed, water is fine, temp is right, fed the correct amount. That's when I noticed her fins. Her tail fin is clearly nipped. It has two solid bites in it with some tearing on the top. I've never seen or heard cardinals killing other fish. I had noticed mine have seemed slightly aggressive, which I originally thought was some small male sparring, but it's pretty persistent. I'm worried about my last guppy but don't know what I should do. I'm also very wary of jumping to the conclusion that the tetras did this since they are normally so peaceful in community tanks. If anyone has any thoughts anything would be appreciated. Thank you!