Ok I've been thinking about the horseshoe crabs and some people said some stuff that I hadn't really thought of. First I assumed they didn't grow super fast super quick, so stinky how huge did yours get in a year? I have an empty 90 gal that I was going to convert into a horseshoe crab tank in the next year. Also I have a 3'x6' tank that in a few years or so will be converted to salt (once the new ray tank is built so we can move those guys to a bigger and better things). I was going to have a snowflake moray that I would obtain small and a lionfish that I would obtain small. I figured by this time the horseshoe crab would be quite large so neither fish would think it was food. Will horseshoe crabs prey on small fish like these when they get bigger? Also once the lionfish and moray got some size on them I was going to add a porcupine puffer (because I've never seen them for sale at a small size), do you think the puffer would attack the horseshoe crab?

And I've never really looked into lettuce nudibranches, it was just an idea I was playing with, why won't it do well in aquaria?

I don't know that much about horseshoe crabs but from what little I have read it would appear that they need a much cooler tank in order to survive. The moray and Lion fish that you would like to add need tropical conditions (78 - 80 degrees) so to me that would indicate that they cannot be kept in the same system.

I agree you should pickup a book and a read a bit or at least surf the forums more. I dont want to be and ass, but sometimes forums are like the blind leading the blind and everyone has a different opinion