Could you be more specific on what kind of "sucker fish" you say you have.
Pleco, Algae Eater or Otocinclus Catfish?
Also, we really need to know the size of the tank as well as what other fish you have in there. That's going to determine what could possibly be the problem.
If you put in a common pleco, and your tank is small. That's why the angel fish died. If this is the case, I'm surprised the plecos survived.
The plain algae eater and otocinclus would cause death if you were already over stocked. The algae eater can get 11 inches while at most the oto will get 2 inches.
We need to know a few more parameters before we can tailor our answers properly.
Add:
Tank size
How long it's been functioning
What kind of fish you have in there
How many of each kind of fish are in there
How much time you waited before adding fish (assuming the tank is new)