CichlidFish.com - Fish That Think.

Undergravel filter? What are the pros and cons?
I have spawning Oscars in a 55 gal.

Undergravel

Pro:
Hides waste

Cons:
Hides waste and gravel vacuming doesn't remove it.
harder to control water parameter (amonia etc..)
if fish dig up one little part they stop working
they can suck down/trap wigglers (baby fish)

There is another reason that it is not a good idea to use an UGF(under-gravel filter).  Oscars like to dig and when the do, they can bare the UGF plate and completely screw up the filter, I recommend a power filter with a bio-wheel, put like fine mesh such as panty hose over the intake tube to avoid loss of fry.

I would remove the UFG and get a BIO-Wheel filter for that tank, but beware, when you do this all the waste that has been acculmilating since you set up that tank will make the water opaque and gross looking.