The identification of piranha is very difficult because there are many common name(nick name) make it confuse.
Moreover many study show the different aspect interms of classification.
This study make me clear !!!!!!!! ..................http://www.aquariacentral.com/articles/bpiranha2.shtml
My conclusion are ( May be wrong)
1 Black piranha and white piranha are the same species which refer to Serrasalmus rhombeus. The different of color is result of the size of fish and different of river.
2 Black diamond rhombeus or Blue diamond rhombeus (common name) are similar to the Serrasalmus rhombeus but they are not Serrasalmus rhombeus. Diamond rhombeus is refered to Serrasalmus spiropleura spicies. This species has sparkle scale than Serrasalmus rhombeus(black piranha). I do not know which one is rare. But in Thailand there are alot of black diamond rhombeus, i have never seen the S. rhombeus.
