Fighting Clowns and Nas. Snails

[HappyHooligan]

The first fish to inhabit my 90g fowler was a brown saddle clown. He had the tank completely to himself for a good month or two until a bristle toothed tang joined him with no problems.
Last week I added a smaller clown (can't remember the exact breed) who is about half the size.
Things were good for the couple days, but today the larger clown was rushing at the smaller clown. No obvious signs of bites or trauma, but some fin splitting and he's obviously hiding. His color is still good.
I shut off the lights and moved some rock around. Anything else I can do? Will the fins heal?

Secondly,
Is there any way of knowing if nassarius snails are okay? Haven't seen either of them for a week or so.