Stocking For A 55Gal?

[simonero]

I'm in the process of re-homing a fish.  When I find a home or place to give it to, I'm going to want to have something else in my tank!  This tank is a 55gal.
 
I have 2 angelfish who will not permit any other middle-top swimming fish in their territory.  They are jerks.  I also have a very passive black ghost knifefish.  I've had him for over a year but he is incredibly slow growing.  He is definitely not near maximum size for the tank, and I don't anticipate him ever outgrowing it.  Hell, even reaching maximum for it.  No commentary on this please - I'm sure.  Other than that, I just a few nerites.  It will be lonely when my other fish is gone.
 
Any ideas?  Ideally a fish that will dwell on the bottom and will not be aggressive towards the BGK, at least not provoked (which wont happen - my BGK is actually buddies with my huge catfish, he's a nice lil guy).  I need a fish that I can buy large enough that a large murderous angelfish won't eat it at LFS size, but that wont get more than 8-10 inches.
 
Thoughts?...  I'm guessing I'll need to order this to an LFS, so I need to know ahead of time.  No tank-shopping possible.  

 
[One idea is a pleco... Not because I have dreams of him serving a purpose in my tank, but because I genuinely like them.  But I'm not sure what types won't get too large, and I haven't had them since before I began real fish-keeping so I'm not knowledgeable about them yet.]

 

[arielsworld17974]

I also have a 55 gallon tank, heres what i suggest. I have a rainbow shark and he is great! Goes well with my angelfish and other fish. Stays at the bottom most of the time, won't get very big (about 6 inches) and they're gorgeous. A black shark will do the same. If you want something more top dwelling gouramis are labrinth fish and like to stay at the surface. They're also really gorgeous.

 

[sawickib]

Really sorry and i know you dont want to hear this, but a normal and healthy BGK can easily outgrow a 55 gallon, i had a 12" and it looked small in 75 gallon, when i upgraded her to a 180 gallon it looked much happier, and ventured out into the open much more often afterwards. I would be more worried about what the BGK would eat, their mouths are much larger than an angels mouth. 
 
A black shark or rainbow shark most likely will fight with a BGK on the bottom. 
 
I like pearl gouramis alot, i have 4 in my 180 gallon and they are really nice, and not aggressive, idk if they would be nice in a 55 gallon though. 
 
I would suggest probably a adult bushy nose pleco.