I'm thinking of buying 2 baby red-bellied Piranha's, but i have no clue how or what to feed them, what they need in their tank, etc.
Here is a list of things you people can hopefully help me with( And that i'll be eternally Grateful for.

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Need to know the cheapest thing i can feed them(Money doesn't grow on tree's in my house, sadly.)
Depends on the size you get them at, most people feed them black worms or froze blood worms or brine shrimps. they are born with teeth so if you get them at around 1-2" or bigger you can actually start feeding them small piece of fish fillet or shrimp. That's what I fed my caribes when I got them at around 2". I'm not sure the prices on those different kind of food but I figure that the fish fillet and shrimp would be cheaper than black worms/froze blooneed d worms/brine shrimps.
Need to know how much to feed them each day(s).
At the size mentioned above you need to feed at least three to four times a day. Feed enough so that they will consum in few minutes or so, if you feed small piece of shrimp or fish fillet they'll stop eat once they get full (should be same with worms or brine shrimp) then you just have to remove any left over food from the tank.
Need to know what equipment i need(Filters, etc)
Piranhas are messy eaters and produce a lot of waste so you need to over filter, i would say at least 3-4 times the turn over rate of the size of your tank. for example if you put them in a 50 gallon tank then preferrably to have filter that can handle 150-200 gallon tank.
It never hurts to over filter so if you can, max out the filtration system.
And around their growth rate, so i know when i'll need a bigger tank
RBPs grow about an inch a month until they hit 6-7" then slow down.
What size tank you plan to put them in? personally I'd like to start them in a tank big enough to house them for live, everytime you transport them you'll be stressing them, less moving is better IMO.
RBPs can grow up to 10-12" under proper care but that takes a while so if you cannot start with a big tank now a smaller size tank would still be ok. However, I would highly recommend getting a tank that's 18" wide at least, don't bother with anything that's 12". the extra 6" in width makes a HUGE difference. if you'll be getting just two then a 75g tank would be well enough. in US the standard 75 gallon has a foot print of 48"L x 18"W. don't waste your time with tanks that are hight because that won't benefit piranhas.
And any other helpful information that would help me & i would need to know.
Check out http://opefe.com for more information, by far the best piranha information site.
Here's the direct link to true piranha species, rbps would be the pygocentrus nattereri http://opefe.com/piralist.html
Thank you very much in advance, if i get them i'll be sure to hang around the forums & get to know everyone.