As for the Rhom, what would you recommend being the final size for a single specimen. I'm talking say I run into financial problems and can't get an "ideal" ize by the time they're full grown? What I mean is that the one CFC has can be in a 20 gallon, ideally a 55 would be best. Like a situation like that. Also, then what is the "ideal" size you would recommend? Thanks for all the help Gatorbait
CFC has a sanchezi in a 20g, not a rhom. Personally, I think a 20g is too small for life but that's on a personal level. Many people have kept them in a 29 for life before also and I had one in a 29g also for a while. I'm not saying I'm right and CFC is wrong it's just what we both prefer is all. He doesn't like a lot of extra room for a single fish and I do. That's about the only difference and like I said it's up to you. When I put my sanchezi in a 55g he seemed more outgoing and would swim around more. That's just THIS personal fish and others might be more active and predatory in a smaller tank.
As far as the rhom you asked about, if you could get a 75g now you'd be set for many many years if not for life. It just depends.
They are, like most Serrasalmus, incredibly slow growers. He'll grow from 1.5" to 5"-7" the first year. Every year after that he'll grow 1"- 1 1/2". This is in general btw but happens in most cases.
I've also heard of a 30 year old rhom that never grew past 11". The keeper is a piranha expert so I know it was well taken care of and not stunted.
it seems you've decided on the S. rhom so I'll give you a few basic details about them. When you get one (or a different species etc) I'll give more detailed info)
There are a couple different types from different rivers like in Guyana, Peru, Brazil. They are all the same exact rhom tho. Their coloration is just different.
They have never been bred in a home aquarium so the fish you have will be wild-caught from South America. The smaller S. rhombeus you get the more skittish it may be at first. It sometimes takes a few weeks to months for them to become trusting. If possible, I would get 4" or larger but I didn't want to spend the money so when I bought mine a couple months ago he was 1.5".
They live in the deepest parts of the river and it's dark and they love the current. They can be trained to like light and some have but it takes time. The smaller ones are in the shallower waters. The part of the rivers they're from are not like redbelly's, it's a fast and strong current so a powerhead would be nice.
I know you'll want to try but let me tell you now that in almost every case the rhom will kill anything and everything you try to add with it. If you add another rhom or elongatus one of them is going to kill the other, it's just the way it is. It's not like cichlids where sometimes it will work and others it won't. This just won't work so save your money.
As far as aggresion goes, this is only my 2nd rhom so I can tell you about my first one and what I've heard of others. Piranhas, like most fish, are a mixture. Some you'll get that just want to chill and don't get worked up about much unless you feed him. others are retiring and shy but when it comes to the Serrasalmus I've fiound that they are more on the aggressive side much more than Pygo's. The last rhom I had I couldn't put my hand in the tank unless he was well fed and even then I had to take precautions. I've heard from others that have had the same thing and some that didn't have a problem at all. Mine would slam the glass and try to attack my dog through it and doing wc's I had to have a divider. It sounds bad but you'll learn quickly about your particular fish. You'll know when it's safe and when it's not just by the way your fish leans or reacts to something you do and when you walk up.
When you get him and especially if he's small, you'll notice that when he's scared he will "play dead." He may not be on the bottom but he'll be floating in the water column maybe up top and he'll lay on his side and float like that. They all do so don't be alarmed that he's dieing on you. Most beginners freak out when they see this and it's perfectly normal and a defense mechanism for them.
Feeding- You will most likely have to feed live fish for a while when you first get a rhom. So plan doing it now and before you get your rhom buy some feeders and quarantine them. Use PraziPro on the feeders for a couple of weeks, that's 2 doses. Then when you get your rhom do that for him also. Wild caught fish seem to always have tapeworms or roundworms from what I hear. This one I have now didn't but others have and many people have Serras that do.
Try to ween him off feeders and onto raw shrimp, tilapia, bloodworms and brine shrimp (if very small only), octopus, squid, catfish fillets/nuggets, earthworms or just about any white fish meats. Stay away from red meats and fatty foods. I struggled to get my 1st one weened off of feeders and even had to go with starving him for 5 weeks in an unsuccessful attempt. Just keep working on it. In the wild they go 2-3 months without eating during the dry season so don't worry.
A word of caution tho... I know that I try to get rid of the myths and stereotypes of piranhas and sometimes lean too far of the way of they aren't that mean etc but a very hungry/starving piranha will be that mean, don't trust them or play with them etc when they are starving and you're trying to ween them.
If you have any more questions please feel free to ask as I can go into way more detail if you need but didn't want to bore anyone with more in-depth information.
), I'll be set, and own one. It just could work... 
I guess I got my answer backwards because the poll reads...
