silvertip sharks or columbian sharks

[truckin35]

i have a 40gl tank with an emporor 400. i have two silvertip sharks. i know after researching them. i will need a larger tank soon which i will soon get. what are som other mates for them. i was thinking a dragon goby.
it is not yet brackish. i would like to know how and when i should convert the tank.

[ILuvMyGoldBarb]

Since these fish slowly adapt to full marine conditions as they grow and age, it would be fine to slowly increase the salinity over the course of a number of months. Just be sure to use marine salt.