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Great that you have experience with the P. splendida, it seems kinda rare compared to managuense/festae. How are they aggression wise? Do you think I could keep a single male with a few females?
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I did not find Petenia to be at all aggressive, but since they are piscavores, anything that fit in their mouth was eaten, so at about a foot long they could easily take adult swordtails and mollies, and at full size @ 15"/16" they may end up a little large for you tank, but if you get them young, it could take years, as they are not fast growers. From 2" juvies, it took about a year to reach 10" to 12" in my tank.
The more natural colored ones have a very similar appearance to managuense, as opposed to the pink version.
The problem with plants and Vieja, is that Vieja are predominantly vegetarian, along with Cincelichthys they are the ruminants of the western cichlid world.