So what would you stock a 40 gallon corner tank with?

Okay so One angelfish(what level do they occupy?) and about 6ish swordtails...

Can I get an upsidedown catfish?

What about some tetras?
 
angels seem to tend to stay in the middle/topish of the tank
for the teras i would recommend rummy noses or emperor or penguin tetras.
as far as the upside down cat is conserned tey get about 6" long and i think that he would be too big to be supported by that shape/size of a tank. instead i would stick to cories or loaches
 
I love giant danios, they have a great personaility, very active, they tend to stay near the top

Get some more Angelfish, they look very nice and they would get along great with the swordtail's.
Look around though to fine good ones, some at LFS are very small and don't look that good.
A lot of people bread them so you might be able to get some very nice ones from them.
Look here, they are beautiful

http://images.google.com/images?q=freshwater+angelfish&hl=en&um=1&sa=X&oi=images&ct=title
 
i've seen them in front of me at 6"+ but there are different kinds of catfish that are sometimes called upsidedown cats.
i would suggest a rapheal but they get even bigger that upsidedwn cats. lol they just look soo cool!
you could to a school of otos (maybe 4-5) and a few flying foxes you could get the catfish and the algea cleaning
 
upside catfish would probably be fine. probably.

if you want a showcase fish... maybe a school of elephant noses, a blackghost knife, fire eel?
 
i would do a type of cichlid tank as my community tank is loosing its appeal to me
 
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