why? the op's original questions have been answered and this is where the discussion has taken us.
if i made a thread i'd put it in the piranha section but since we're in general aquaria where a whole different set of eyeballs will see it, i'd rather we discuss it out here, on here
Discuss what? Your opinion on why you think a Piranha is an intermediate fish? Is that your opinion? Or is it a beginner fish? Is a Piranha easy to keep? In your opinion is it so easy, just put it in a 20g, make sure the filtration is good and only feed it daily until it gets to say, 6", then cut back to what,twice weekly? Is it your opinion that that is easy, intermediate or advanced? Sounds fairly easy to me. Fair enough.
But lets take into consideration a few well known facts about the most popular Piranha available commercially, RBP. It gets big quickly. Most people, advanced aquarist alike, dont plan ahead well enough and usually get to a point were they have to much fish and not enough tank. We all have done that, even the pros out there. If you are in this hobby long enough, you might even try to keep very wrong species together for lack of room...but I digress.
Red Belly Piranha are a schooling or shoaling species. That means, as you know, they require companions. Whether or not you agree, schooling fish always benefit from having a school to be in. Whats constitutes a school? I would say common concensus would put that at around 6. Minimum. There is probably a lot who would say 4-5 but I feel 6 is more realistic in terms of quelling any dominance issues. It is well known that single or paired Piranhas are, at best, skittish. 3-4 and they become aggressive toward each other. A single one in a tank? Who knows? The potential for aggression from this fish, especially a skittish single...I wouldn't want my little kids putting their fingers in the tank...just sayin.
What to eat? Fresh meat. How fresh? Frozen? Live? I dont know. In the wild what do they eat? Frozen or live? Do they eat plants? No, meat. Carnivore....a fish after my own heart. Feeding a fish a specific diet doesnt necessarily make it advanced, but one could argue that a specific diet makes it a chore none the less. Are you really going to feed a Piranha pellets?
So, schooling fish of size requiring not only a spacious aquarium but special feeding requirements and handling techniques. Oh yeah, special handling techniques. I got bitin by a 12"RBP that went to the bone. Didnt even hurt. Teeth were so sharp, the bite didnt even bleed for several minutes. Then it was like a murder scene. Scared and scarred me. Scared me because I realized that this fish could have easily removed my finger. Yes, these fish require special handling and if that isnt all, they are able to jump out of their tank so then they REALLY need special handling. I dont know about anyone else, but a thrashing fish on the floor with scalpel sharp teeth is something for an advanced aquarist to handle.
So, as I see it,(through my eyes) Piranhas are best left to the advanced keeper. There are always exceptions to the rule and I'm sure you will state these quite eloquently, but the truth is, many attributes of this hobby are trial and error, but most husbandry practices are tried and true. Gold standard stuff. I will follow the advice of people much smarter than me and choose not keep a singular Red Bellied Piranha (in a 20g or 200g), and I certainly won't encourage irresponsible fish keeping just to answer a question of semantics.
