Well, my Elongatus as stated in my signature is in a 100 Gallon tank with an Amazon tank setup. which entails some live plants. I keep the temp at about 80. PH I keep about 6.8 - 7.2. I've not had any problems. They are solo fish and can not have any tank mates. Feeding is not hard, I give mine feeder guppies, roseys, and goldfish. It's not a difficult fish to keep kicking. Just do a little research on Piranha tanks vs average freshwater tanks, there is very little difference. they just like the water a lil warmer and the PH a little lower
I will have first one of Serrasalmus family, but I don't know, which is easier for care, black piranha or elongatus (pike piranha)? I have 6 red belly piranhas. A tank will be about 50gallons?
A tank of 50 gallons is going to be too small for an elong. They do not get that large but they are very fast and agile swimmers. They will need the room of a bigger tank to swim. You are better off getting a black. I would say elongs are a bit more difficult to keep then a black. Again though depoending on the actual specie of "black" you get ( there is a myriad of different species importers and LFS label as "black") your 50 gallon will be too small.
their care is basically the same. the elongatus is more likely to be more active then the black. they dash around alot, so a tank with a good length is nice to have so the fish wont bump around.