Blackfin shark/catfish

Question
QUESTION: I recently bought several fish, two of which include a blackfin shark and a pimelodus pictus catfish. I had thought that both of these were bottom of the tank fish however, ever since they have been in the tank they have stayed at the top of the tank except during feeding. If they come down at all it is for a very short time and they also occupy a very small area of the 50 gal. tank where they pretty much swim in circles or back and forth. Is this normal behavior?
ANSWER: Hi Christopher

Yes, those fish are mostly bottom dwellers.  I've had pictus cats before, and they would swim back and forth.  I don't recall if they swam in circles though.  It's been awhile.  I had moved to a new house with well water, and they died within minutes of me acclimating them to it.  Something in the water just didn't sit right with them.  So that tends to make me think they're a bit sensitive to water conditions.

What else is in the tank with them?  Any larger fish that may be scaring or harassing them?  
How long has your tank been running?  
Is it completely cycled?  
Do they look like they're gasping for air at the top of the tank?

Both of those fish get rather large, and you may need to think about a larger tank for them.  Another thing, I remember reading somewhere about Columbian sharks being brackish fish.  If that blackfin shark is the same as a Columbian shark(which I think it is), they require a brackish water as they get a bit older, and some say full marine when they're adults.  Here's a link with more info on them, and the second link has info on both catfish, you'll need to copy/paste to your browser:

http://badmanstropicalfish.com/profiles/profile63.html

http://www.liveaquaria.com/product/scateg.cfm?pCatId=1163

Compare the photos to make sure that is what you have.  If so, might be best to return it to the store, unless you're willing to set up a large salt water tank for him.

Let me know on those questions above.  Also, I'll be out of town most of the weekend after tomorrow late afternoon.  I'll check in here before I close out my section here.

Christy

---------- FOLLOW-UP ----------

QUESTION: Great answer! To answer your questions... Yhese are the largest fish in the tank. I have a pair of Black Mollies,a pair of Lyretail Mollies, a pair of Red Swordtails, and 3 Cardinal Tetras as well as 2 Blue Mystery Snails. The tank was completely cycled, and has been up for about 2 or 3 weeks now. They all look very healthy and not gasping for air. We have added some salt to the water as one of the Lyretails turned completely white (from orange) and further research showed they also like brackish water. We havent added a lot just yet as we are still looking into the other fish. Dont want to add too much and cause problems for the others.

Answer
You probably know, but you're going to want to watch the salt with the snails.  I tried looking around on the pictus cat, can't find any info regarding them and salt.  A lot of catfish are a bit salt intolerant as well.  So keep an eye on that one also.

From what you said, it sounds like everything is doing fine.  As long as the water parameters check out, the fish and act healthy, and are eating.  That is strange they're at the top of the tank.  Do they have caves to hide in?  I still can't imagine that making them hang out up top.  Here's a thought, is the water circling through the tank good enough?  Could there be a temperature difference?  

Try posting this question with a different expert, maybe someone will have more of a clue then me lol.  

Good luck with your tank and have a great weekend!!

Christy