K is right. Read the stickies, reread them then read them again.
If you're going to do payara successfully, you have to make sure that you specialize the tank to their needs. With the exception of Hydrolycus armatus, the other five species of fish commonly called payara (Cynodontidae et al.) are all very slow growing and to see one at 18 inches or larger is truly rare and exceptional. Again, with the exception of armatus all of these fish are fairly able to be kept in a community of other large, peaceful predators; if its too big to eat they're unlikely to bother it.
Armatus are a different story. They can get huge. Three feet is not unheard of and really exceptional specimens can reach over four feet in length! Granted very large sizes such as that will take some time to achieve, but these fish can hit two feet in captivity in about a year with proper care.
The main problem you're likely to encounter is that armatus that they get huge, have an attitude to match their size, and they have the weaponry to back it up! I love armatus, but I am leery of having one, as they can and have killed just about anything that they can be kept with. Some are comm'd well, some are murderers. If an armatus gets hungry, it also has the temperament and equipment to eat fish a bit bigger than half its own size. There's a video on youtube of a foot long armatus who ended up eating an entire school of Cichla orinocensis! Talk about expensive feeders! They are great, rewarding fish, but I wouldn't keep one with anything I cared about! The trick for being successful with armatus seems to be adding a young one to a tank full of larger, mature fish that the armatus won't mess with, but if you do that you run into the problem of the armatus becoming a snack. Way down the road, if I add an armatus that is how I intend to do it.
As for the care, I would choose the 300 gallon for your payara regardless of the species you choose. Set it up with powerful currents and places for them to hid to get out of the current. These are riverine fish that live in fast, highly oxygenated waters that have a lot of roots, rocks and other detritus on the bottom and sides that allow them a rest if they so desire. For these fish to truly thrive, you need to imitate this in your tank. At times, payara like to swim in the current, I've found that this is often feeding or foraging behavior for my fish. When they're sated or scared, they tend to hang out in the calmer areas of the tank. For a 300, I would recommend 6000 or more gph for the fish. Two Koralia Magnum 8 Powerheads at the top, pointing longways to make a 'white water' effect on the surface would likely do that well. More current is always good. I'm running 5000 gph in my 150 growout and I plan to up it by an additional 6000 gph or so as finances allow.
Hope that helps lol
