Swollen doesn't mean its egg bound. If I were to make a guess the swelling is due to over eating and some sort of blockage or fatty liver of some sort. How old are the fish? Like I said before you should try raising out pacu to adulthood before trying aquaponics. You will soon learn that they are not a good option for aquaponic systems no matter how big they are.
And I've already told you the natural habitat. Please do me the favor of reading my posts as I have yours. They live in the amazons. They come from South America and are in found in the amazon rivers. To stimulate breeding for fish from amazon it normally requires a large water change and temperature drop. More specifically you have to make the water very soft and have the temperature drop for a short period to stimulate rainy season.
Goodluck finding more info then that. If there are more knowledgeable members they won't be sharing their knowledge knowing what you plan to do. Even me, who is big on defending overstocking, will not say this is a good idea. Grow them out to eat. Find another species to breed.
