that's s. maculatus... they are one of my favorites, i have a couple myself. a 75 gallon would be a huge tank for a small mac, but if you don't mind having one smaller fish in there, i'm sure it'd be happy as hell.
I'm really liking them more every time I look at them. I want to find a purple spilo somewhere, I've wanted one of those for a while. might get 2 golds...
a mac cohab is a pretty big thing to undertake, i wouldn't recommend it unless you have alot of experience and you are able to keep a very close eye on them and have tanks to move them into if/when you end up needing to separate them. if you want more than one fish in the tank, a few natts or caribes will be your best bet.
I've got a couple empty tanks right now. I'll probably just get one for now until I can get my big tank and then get 2 or 3 maybe. I want like a 150 at least...
Small Piranha but doesn't have a solid common name or much of a taxonomy. However they are popular in pet trade as Red Diamond Piranha or Ruby Spilo. Picture/Info courtesy of wwww.thejump.net