Although I've never kept one, I believe they don't grow as large as most other South American cats. Even if they grow to 24" your tank should be fine for quite a while if not for the life of the cat. They are thin fish unlike many others and from what I see are not very active fish either. I say go for it, you tank is probably bigger than most keep them in anyways. Bigger is always better thats a given, but a tank that size should suite that fish just fine.
JMO of course............
And I've never kept one either........
bob