I have a planted 36 Gallon tank with the current stocking: 5 Sterbai Cories 1 Male betta 2 Platies 10 Rummynose Tetra Last night I had the death of two of my cories. The first one I just found, upside-down on a leaf, dead. NOTHING looked wrong, he didn't look bloated, pale, nothing. If anything his colors were darker, but I don't exactly remember now. I was disappointed, but I didn't think too much about it. I did a 35-40% water change just in case. Fast forward a few hours (It's around 9 pm), one of my cories darts around the tank, by the surface most of the time, and then goes to the bottom of the tank belly-up. I take it out at this point. For a few minutes it seemed okay, but then it becomes unresponsive with small bursts of activity, often floating belly-up. He seemed in really bad shape, and with the death of the other one, I euthanized him with clove oil. I didn't think to test my water last night, but after I did another water change today I thought to test it. 0-.25 Ammonia 0 Nitrite 0 Nitrate I don't know what happened, my tank has been set up for a while (and cycled) just fine. 2 weeks ago or maybe less, I got 2 cories, 2 platies, and the rummynose tetras. I know one of the new cories was one that I euthanized (he had messed up stripes, so I recognized him), but I'm not sure about the other. What should I do? None of the other cories seem to be behaving oddly, everyone else seems fine. I don't want to lose anymore fish.