[redpaulhus]

Pacu and "planted tank" should never be in the same sentence - its more like "cow tetra and salad bar".
Pacu and 75g should never be in the same sentence.
"Oscar with live plants" usually translates as "Bull mastif with chewtoys" -- most adult Oscars will uproot and carry around most common live plants.
Oscar should only be in the same sentence as 75g if "oscar" is preceeded by "one" (and "water change" is preceded by "weekly" )

You could spend $5000 or more on filters - it wouldn't change anything, the tank is too small, and ALL filters simply turn waste into nitrate -- only water changes (and large volumes of water to dilute the nitrates) keep nitrates in check. The cascades will probably be physically sufficient for a little while, but they may become biologically challenged soon, as your little babies grow (your "big" pacu probably hasn't hit puberty yet).

There was a great series of articles in either FAMA or TFH in the 80's, about a single pacu who outgrew a number of tanks (450g, etc) til he was in an 800+ gallon tank that needed to be built in-place in the basement of the house because it would could not be brought in thru any of the doors.
That was for a single pacu.