a bigger tank is always better, more water volume = better water quality (more tolerant to nitrate build up ect.) = better health/growth, etc and manueli is one of the most delicate piranhas so you want to keep water as clean as possible.
you don't NEED it now but if you can provide one it'll be better in the long run and you don't have to upgrade again and again,etc.
with a bigger tank you also get more space to decorate the tank to make it more appealing.
here's a video of one of the best manueli I have seen and it's in a 250 gallon tank with 3D background and live plants. the owner had it for more than 6 years and it grew from couple inches to this monster size.
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