A very loose guide for Red bellys is 20 Gallons per fish.
Though in a 200g IMO 7-8 would be max id put in, not 10 as the rule would suggest. The idea of a guide is just that, its then for the individual to "fine tune" as every set up is going to differ slightly and every Piranha has its own "personality" (behavour/tolarance factor)
Although in my 180g(uk) i have just 2 rbp and 1 large toothd piranha.