I need some advice and experience to step in here...... I decided to build a tank without any mechanical filtration. Though I am still filtering until the plants grow. But this tank is a feeder tank. It has gambusia, guppies & mollies. I had the worst outbake of fin rot....so I originally thought. The fins on the gupps disappeares. 75% of the gupps were without fins and or dead in the a.m. These are/were fancy gupps. I am treating with a broad spectrum medication so I can halt this epidemic. So, I look this morning for the kill count. I saw a few gambusia chasing and nipping at a gupp. There's the history. So is it sickness or bullying. Has anyone kept gambusia and seen that they are bad little nippers? Was it just a coincidence that I saw a nipping?