You might be able to keep one for a year or two. If you're 100% going to get a bigger tank later on, you can keep one. Although they have potential to grow up to 17" or more depending upon the species, but most won't even reach the half of that size in captivity even under the best conditions. (The ones in public aquariums are a different story). Anyhow, my point is...if you start off with a small porky, you should be fine. Just don't be tempted to overstock your tank...