DRAGON GOBY

Question
I just bought 2 silver tipped sharks,(columbian sharks) the pet store said they grow to 12 inches but i read up to 20, i have them in a 40 gallon long tank, just the two of them, and i was told after they reach 8 inches to turn the tank to brakish water. so right now i just have the tank with aquarium salt as i was told they need while young. so heres my questions:
1> are the columbian sharks compatible with a dragon goby,
2> is what the pet store said right about the salts
3> how big do they really get
4>what are the ph levels for these fish and if the goby is not compatible what fish are, i heard mollies are, but the petstore said that there not really compatible with gobies/
any quastions or all that you can help me with will be apreciated thanks

Answer
Hi Chevy,
  The "columbian shark" is a catfish of the genus Arias.  They do grow very large (possibly a couple of feet).   You need a MUCH larger tank for a fish like that (100+ gallons).  A fish that large is not going to be compatible with much and my guess is that the dragon goby will quickly be eaten.  Also be aware that many catfishes can "sting" you or other animals with the tips of their fins (sometimes it is the dorsal and sometimes it is the pectorals; I don't know for this particular species).  All in all, it is not a good aquarium fish for most people.  
As for the salt, the fish lives in estuaries (brackish water) so having some salt in there is necessary.  

-- Ron
  rcoleman@cichlidresearch.com
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