35 gallon...

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Although it's not predatory, bumblebee gobies are cool brackish fish. You could get 3 - 5 of them in there.
 
For a brackish tank you will need salt of course, and you should get a hydrometer to measure the salt levels in the tank, it really isnt much more difficult than full freshwater except for measuring salinity and adding salt, so for sure pick up a hydrometer.
 
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