if it wanted it could probibly make a small wake LOL... my 14 inch oscar makes HUGE splashes and it is like half your pacus size... I don't know if a pacu would jump out of the tank so get that lid quick... he could probibly break the top and jump out if it wanted though...
if possible, stablize the ph by adding some things like limestone type rocks or crush coral and not let it drop any further. While low PH prevents ammonia damage, it also inhibits bacteria growth; but don't use any chemicals to artificially raise the PH though.
What is the PH of your water source? One thing you don't want to happen is through a water change, raise your PH drastically and suddenly you have toxic ammonia in the water.