Seeing them shred things apart is fine and all but there are many other priorities with a shoal of piranha. Imo if you get everything just about right (filtration,water quality,stable temps,cycled tank,nutrition etc) then observing their natural behavior is a reward and the more healthy/natural you maintain the tank the more natural behavior you get to watch!
Setting up a 150gal with a dozen piranha needs to have a serious checklist and the first box that needs checked is research! A lot has changed over The years with FK and piranha keeping in General so look thru tons of old threads/stickies and advice
While you research you can set up the tank to cycle properly so that the nitrogen cycle is complete when you add the piranhas. That will ensure a healthy start, plus ammonia is MUCH easier to deal with when there's not a dozen hungry piranhas in the tank.
Just grab 1-2 fx5 filters or a sump (add a decent amount of bio media) grab 1-2 300watt jager heaters (preferably with a heater controller) buy some prime water conditioner and add tap, then throw in 15 or so goldfish. You can buy an API fw master test kit for under 30$, test every few days along with small 1-2x per week water changes. eventually ammonia will read 0 then nitrites will do the same, at that point you can add the fish. Like stated above babies are a good start, they grow quick and seeing them grow up healthy and strong is a great feeling.
1 fx5 or eheim 2262 will be fine for juvies but after 4-6 months you will need more filtration. They prefer dim light and good hiding spots, they will be less skittish
Take it step by step. Impatience and rushing will only give you big headaches down the road. Proper planning now is vital!
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http://www.monsterfishkeepers.com/forums/showthread.php?504763-Cheap-plants-less-nitrate!-POTHOS
