What should I do

I have a 55 gal tank with (roughly 30-40 lbs) of live rock, sand, 2 orange clown fish, and a serpent star fish. The tank is environment is roughly 2+ yrs old. I have an ocean sun light, a protein skimmer, and a powerhead. What else can I have, (fish wise) in there with out upsetting the nature of things? Also what food is good for the new fish? My tank photo should be on my profile. I used to have feather dusters, but there gone now, and my hermit crab was eaten by what looks like a whitish brown umbrella. Can anyone help me out please.
Sedb35.

 

Whitish brown umbrella sounds like a Polyclad Flatworm?
Your clowns will make introducing new fish a challenge, it's their tank now and intruders will be dealt with harshly. You may need to put them in a breeder box or sump while new fish acclimate for a few days.
You can however move the rock work around just prior to a new introduction to establish new territories. Keep lighting off during introduction as well.
You'll want a fish that can stand up to two mature clowns. Dotty Backs come to mind as do Dwarf Angels (not potters or coral beauty), Bi-color or flame.
Other damsels like the Blue and Yellow tail.

 

missing equipment

I dont have a sump or breeding tank. I googled flat worm, im not sure thats what I have. The plant or whatever it is looks kinda translucent with a stringy flower type head. So you would advise not to integrate new fish, for it may be dangerous to the fish? Have you seen my pict? What else could go in there, snails, crabs, shrimp, or sea horses? I appreciate your rapid responce eariler, and looking forward to any other advice. Bye. Sedb35

 

Sure you can add more fish but be aware that clown are damsels they stake out territories and defend them often killing intruding fish that can not escape. There are items called acclimation boxes or breeder boxes that you can buy, they go in the display and hang from the side. To introduce fish you're going to need to change up the rock work (based on your image that seems very doable) and perhaps remove the clowns while the new fish acclimates for a few days. Get fish that are not passive and can stand up to clowns, no young fish either. You can add fish, you just can not go purchase them and add them to the tank and think it will all be fine, chances are good the clowns will be aggressive.
Ideally Seahorse tanks should be cooler than the average reef tank, Seahorses can not compete against clowns for food. Seahorses are definitely not a good idea for that tank currently.
I need a little more information regarding your lighting, I googled that brand and did not come up with anything. Lighting type ie: LED, Power compacts, T5......
To get an idea of what your lighting will support I need type of lighting and any other information like watts and so forth.
I am sure we can come up with a number of things you can add to the tank.