Today one of my fish died (oto) when seemed completely fine yesterday and now my female swordtail is at the top, seems to be having a hard time swimming/moving and her belly is strangely bloated not just round and out there but kind of out of shape bloated. The only thing that was done differently yesterday is that I put a "tetra tank nibbler" in there (the veggie ones that stick on the glass) which I've tried a few times but usually end up taking them out shortly after. What's going on? What should I do? Was it possibly the tank nibbler? Also, my tank is fully cycled. Any response is much appreciated!