im also pretty sure acetone has a lower density than water, not just a little less dense, but light enough to separate on its own. Its likely that a lot of it sat at the top of the tank instead of mixing into the water and hurting your fish. That being said, your running an acrylic tank which is susceptible to melting, perhaps your nail polish remover had different chemicals in it? I would imagine any amount of nail polish remover to leave melt scars on the tank.
I once put acetone on a piece of paper and rubbed a stain on a capet the carpet melted, its pretty caustic stuff..