SM> I've just aquired a fairly large tank (roomate moved) 
 SM> with a couple of angel
 SM> fish, a sucker of some sort, and a few sword-fish ?
If the "sucker" looks like a small ugly catfish, he's probably a plecostamus 
(or pleco).
 SM> What are the most important things for me to do ?
Making sure the filtration system is working properly, and maintaining a 
consistent temperature are very important.  Your roomate probably had the 
heater setup already, but check the temp.  I'd recommend about 80 degrees F 
for these type of fish.
 SM> How often should I feed them ?
With these fish, probably twice a day.  As much as they will eat in about 
five minutes, or until they stop feeding, whichever comes first.  Also, the 
pleco will enjoy an occasional treat of boiled zucchini.  You can drop in 
slice every now and then, and he should gobble it down.
 SM> What about water replacement.
Make sure that you have a watr conditioner/dechlorinator.  Pure tap water can 
be harmful to your fish.  Add water as neccesary to keep the tank 
basicallyfilled.  About once a month or so, you can drain 25%-30% of the 
water and replace it.  When you do this, you should get a tool that will draw 
the nasty junk up from the gravel when you drian the water.
Aside from that, it's pretty simple.  Also, try not to leave your tank light 
on too much, as that causes algae to grow.  You want some algae, but so much 
that the glass gets gross.
Good luck,
Robby
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