I aggree with the others.
It will be next to impossible to keep those fish happy in the tank they are in now.
Like Win said once: No matter how hard you hope or pray, those Oscars will keep growing!
Neither will a Internal filter do you any good for 3 Oscars.
Internal filters may be good for some mechanical filtration and for creating some current, but for bio filtration you will need a big canister filter to go with the new tank.
You need an upgrade to keep your fish happy, healthy and alive.
Keeping them in a tank that small will give waterquality problems no matter how religious your waterchange scedule will be.
Although this may not be the advice you expected, please let us know what your future plans are!
