To echo the above, a MUCH bigger tank!
Also, you would be talking at least 2-3x extra filtration - there's no way a Juwel internal would cope with adult red bellies, but that's OK because Juwel don't make any tanks big enough for keeping adults anyway.
These are very large (a foot), messy schooling fish that are very social with each other so should be kept in groups - but they are also very likely to injure each other frequently through minor nipping (just like tetras - but these guys have bigger teeth). To spread out the aggression so that one individual isn't constantly being picked on by the others, you need to keep a good size shoal, so lets say 8 as a minimum (though this is conservative IME). 8 x foot long, nippy fish need a lot of room - think of something more like a 5'x2'x2' tank with 2 or 3 massive external filters. No budget options here unfortunately, the water needs to 100% perfect as these fish will have cuts and minor wounds frequently and you don't want anything getting infected...even better would be a sump.
To top it off...while beautiful fish with awesome jaws...they are boring. Basically oversized tetras, they just sit around the tank most of the time not doing much until you feed them. Feeding isn't even that exiting either, it's bad for their health to give them live fish and in nature 50% of their diet is plant based, so they really do best on regular tropical fish pellets mixed with frozen food and some greens (cooked peas, aquatic plants etc.). So pretty much the same as regular tetras there too.
Most here would highly recommend the Tetratec external filter range, the best filter and best value option for your tank would most likely be a Tetratec ex1200 - around £90 shipped
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For reliable heaters I would say get a 200w Visitherm, Fluval E or Eheim Jager heater - all very reliable models IME. If you prefer something hidden and out of the way, you could try a Hydor ETH external heater, these fit in-line with the external filter hose but are about twice as expensive as regular internal heaters.
Given the above, however, you may want to rethink the piranhas.
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With 4 your just asking for one of them to be bullied...and then another and then another. You would be far better with just 1 in a 75 gal, it would be lonely but at least it would be safe! 
