Almost all piranhas are extremely hardy, with manueli being the exception. The bigger issue with manueli is that they do not ship or transport well at all. It is the reason there are so few large ones in the hobby. Large ones almost always die when being shipping in SA, and many people lose their manueli when they move them. I never kept one long term but lots of people talk about them needing clean water and being sensitive. Rhoms can take a beating. I kept one for ~18 years and I probably moved him 10-12 times and had many issues with his tank over the years that he always survived.Do you mean that manueli need better conditions to survive while rhoms are hardy?