Mix some vinegar or bleach in some water and scrub it really well. Make sure to rinse it out really well! If you use bleach, when you rinse it make sure you don't smell any of the bleach when your done. If it isn't really dirty, you could probably just use water.
As far as the fish you are asking about, I wouldn't put any of them in a 55gal. Too small for the fish IMO. Remember when you are looking at fish to keep, take in account their adult size and growth rates. Lions and triggers grow pretty quickly and can reach over a foot. As far as anglers, I don't know much about them. Be sure to research your fish before buying. Read up and learn their needs. Lions are sometimes difficult to keep because most are hard to get adjusted to eating flakes/frozen foods. They usually are all live food eaters. Most LFS just feed them goldfish (horrible diet for them) just to keep them alive. Triggers are great fish and fun to have, but like I said they get large quick and very territorial. HTH.