If your fish(guessing they're rbp's) are just teenage they're probably around the 4" to 6" mark, they're still small if so. A standard 90g with 7rbp, and a 10pelco... I don't think it's fare to the fish to be adding anymore to an already crammed tank. On the Piranha forums people try to cohab P's with other fish all the time, and over time it's always the same out come.... The amount of time it takes to get to that point varies, it's basically a hit or miss.
What are you using to filter that tank?
Thanks for the responses...as for crowding, they seem to be doing well. They are very cool just hanging out in their group...slowly swimming in and out of the fake plants, driftwood and rocks.
As for filters I have 2 Fluval 205's an AquaClear 30 and a powerhead. Quite a lot of water movement really. I also clean the tank once a week.
I introduced about 20 small feeder goldfish into their tank...and as I was sitting on the couch watching, I liked the idea of having more movement in the aquarium. However the goldfish didn't last more than 10 minutes...so I was thinking of a smarter, faster, more wary fish. But yeah...chances are anything I add will just end up as food.
Oh and yes they are red bellies. Very attractive
I do have a 125 gallon they could go into...hoping they stop at 8 inches and not go to 12...guess we'll see...