Onyx softshell turtle

[My Head Hurts]

Okay, my LFS got these in and i want one! ive done a bit of research but i wanted to ask you guys a few things....
Im thinking of keeping one in a 14"8"12" tank, hopefully it will be moved into a larger ank when it gets fully grown, but will this be ok? How fast do these things grow roughly? Also, my LFS guy says they should be ok at temperatures down to about 20C, but would this be a bit cold?
cheers!

 

[Oddball]

its a turtle so at lower temperatures it goes into a state of torpor becoming slow and sluggish, this is alright in itself but could drowned it if it cant keep its head above water!

 

[Timmy]

Not sure what you mean by 'Onyx', do you mean Trionyx?
Softies grow quite fast if treated right and they like a lot of open swimming space so your tank sounds small unless you plan to upgrade very soon.

20C is cool for a hatchling, though there are many softshell species so maybe OK.

 

[My Head Hurts]

yeah, i meant trionyx, the guy at my lfs says try "insert fish name here" if you want to search for something, so i got confused!