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
Dear Chevy,
1. dwarf puffers and scats, you keep with dragon goby and columbian sharks too.
2. they are right, you need to make the water brakish.
3. they can reach up to 24 inches.
4. 6.5 to 8.5 ph level.
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