I've never heard of an aquarium spawning. I've only heard of captive farm spawning.
Pacus spawn from Oct to June. Females lay from 59,000 to over 400,000 eggs. Pacus are egg scatterers and avid egg-eaters. So, if you were somehow successful in spawning them, in aquaria, you'd have to remove the adults after they're done spawning.
Pacu farms coordinate the spawn of their broodstock with introduced hormones so all females lay eggs at the same time.