Iv had a red belly piranha tank before and the main thing you need to know before going ahead is that they are not what you are probably expecting lol most people believe the hype that they are killing machines... not at all true. They are in fact very shy and somewhat boring. I had a group of 5. They would swim away from me when i put my hand in the tank. This is a common question amongst ppl looking to keep piranhas... how do you clean the tank with them in there? Well... as i said, you put your hand in and they swim the other way.
They will only attack something if they know they can kill it. Or if the prey is already wounded. They need to be in groups as they wont hunt otherwise. Again they are not strong unless in groups. Unlike big cichlids.
Piranhas have next to no personality and spend alot, if not all their time sat at the bottom of the tank.
They need dim lighting, again as they are so shy. So they also need alot of cover too. They are very messy eaters as well so be sure to have ALOT of filtration. You will need around 10x your tanks water volume as a turnover per hour. So as an example... if your tank is 250 litres then you will need a filter thats will turnover at least 2500 litres per hour but more being better. Also get ready to syphon the uneaten waste off the bottom of the tank around once every 2-3 days as well as your weekly 50% water change.
You will need a tank of around 450 litres or more (100 gallons+) for a group of 5 piranhas but you will need to upgrade to a bigger tank. They get big... a 5x2x2 would be a minimum for 5 imo. That is the size tank i moved mine into. They were in a juwel rio 260 (4 foot 260 litre bowfronted tank) but they soon outgrew that.
Anyway.. my group soon went from 5 down to 1. I had that 1 piranha that didnt like his own kind and he killed the rest of them. I then decided to rehome him and get a blue thai silk flowerhorn. Best decision i made. Even though he we was much much more aggressive than any of the piranhas meaning i had to keep him alone in a 5x2x2 tank. He had so much more personality too. He would swim up to the front of the tank when i walked up to it, he would swim around in circles to get my attention... as soom as he seen me looking he would stop doing it lol. Cichlids are better by far but if your still stuck at getting piranhas then go for it
i suppose they just disappointed me after the a while. I got bored of constantly syphonin up uneaten waste... wasn't my idea of fun lol would of made up for it if they were rewarding fish to keep but they just weren't
ah well... im still kind of happy i gave them a go.
If you were to get a the 5x2x2 tank needed to keep 4-5 piranhas then i would run it with 2 fluval fx5 external filters. They are by far the most hideous filters iv ever come across but they are also 1 of if not THE most powerful so they would be need for a group of fish as messy as piranhas.
By the way.... that link states you can house 4 red bellys in a 75 gal tank!! I would definitely disagree with that. A 4x2x2 which is 100 gal i would say you may get away with but id go with a 5x2x2 like i said above. They can get to around 12 inches long and deep in the body so a 5fter would definitely be better. Even a 6ftx2x2 if you got a group of 6