As an owner of three Pacu and one RTC in their "juvenile" stages, (RTC is about two feet and Pacus are from 14 to 22 inches), my opinion is that these two species coexist very well. If you train the Pacus to eat from the top of the water, and the RTC to either eat from your hand or pellets that sink, you can avoid a lot of competition at feeding time. Keep in mind that the Pacus are going to eat whatever goes in the tank, even if it sinks to the bottom, so hand-feeding your RTC is really the best way to make sure he gets enough food and the Pacus don't explode. I keep my guys in 1,000 gallons. IMO, this is the smallest size appropriate for these monster fish as adults. Also, I'm going to suggest that, rather than buying tank after tank to grow them out, just go ahead and build the final product. Chances are, nothing you purchase in the process of grow out tanks is going to be useable on this final product, and we all know you can't get your money back out of used products. This large volume, along with the opaque sides which will likely accompany it (unless you're doing a 1,000 gallon aquarium... in which case, can I have too? LOL) will solve your Pacus' skittish problems. Mine hardly ever "freak out" in the manner that I've seen Pacus exhibit while in an aquarium.
1,000 Liters is about 250 gallons... not large enough to house these guys for more than six to eight months, I think.
