I can get a 4-5 in rhom and I have short time to decide....the only thing is a have a 20 gallon tank around....how long can i keep in the that tank. I will need to save some cash in order to get a bigger tank.
I currently have a 4,5" Manueli in a 15 gallon (dimensions are 24x12x12"; same situation you have - short notice, no time to get a decent tank first), and his doing fine so far: he eats well, and is fairly active.
I'm planning to upgrade soon, but I think a fish of this size would be fine in a 15-20 gallon tank for a couple of months...
I think that the 4-5 inch rhom will be okay in the 20 for a while. Perhaps a year or so or whenever he hit about 6 inches. A 20 Long would be alot better though. Good luck with your new fish, and keep us updated.
Dunno: I had reds in that 15 gallon before I got my Manueli, and they had a normal growth rate (about 1-1,25" per month).
If you compare the size of the fish and the dimensions, the short sides are more than twice the size of the fish, and that's more than can be said about most large tanks with larger fish, so I guess as long as you keep the water quality top notch, offer a steady diet and some excersize in the form of a powerhead, he'll be fine, until he really gets cramped.
Just keep in mind that a small tank is much more vulnerable to ammonia, nitrite and nitrate spikes and other contaminations, so keep a close eye on the water condition.