question about sharks.

it's a Hexanematichthys seemanni. Colombian Shark Catfish, Shark Catfish, White Tip Shark, Black Fin Shark (the different names they go by). I usually see them mixed in with the pictus catfish at petco. As soon as you described what it looked it, it sounded a lot like those...grayish body with black fins. They usually get up around 13", but another site says 24" . Here's a site with a little more info on them:
<a class="postlink" href="http://animal-world.com/encyclo/fresh/catfish/sharkcat.php" onclick="window.open(this.href);return false;">http://animal-world.com/encyclo/fresh/c ... arkcat.php</a>

 

another site with more info about them:
<a class="postlink" href="http://www.fishlore.com/profile-columbianshark.htm" onclick="window.open(this.href);return false;">http://www.fishlore.com/profile-columbianshark.htm</a>

 

Yea, one site says you can start them out in freshwater, but as they grow you have to go from fresh to brackish, to full saltwater, and it's best to have them in a 75 gal not only because of their size, but because of their swimming habits as well.

 

75 at full length?

 

towards the top of the one site, it says
Care Level : Moderate, needs a very large tank as adults
and then below that
Tank Size : 75 gallon (284 liters) minimum for one, much larger for multiples. This fish needs a large tank not only for it's size, but because it is a fairly active swimmer.
So you could probably start out in a smaller tank, but you'd eventually have to upgrade to a larger tank and along with it, slowly switch it over to saltwater.

 

If the fish does get bigger than say 14" I think a larger tank is neccessary, a 75 is only 18" wide if I remember correctly!

 

+1 on that... fish do not "grow to size of aquariums"... they grow to filtration capacity of aquariums (and slow/stop growing then because growth and lifespan are stunted due to diminishing water quality) I've known many people to buy gorgeous small specimens with full intention of getting larger setups when the fish needs it - normally that time comes and goes long before most aquarists (myself included) can tell that the fish is developing issues. Much safer/happier for everyone involved to meet living requirements and psychological needs.

After all, we could survive in a bathroom our entire lives - it's large enough for us to move around and we wont outgrow it... I would personally hate to do that to a fish.