the other way u can get the black background without an over head flash is
if u have enough lighting on ur tank, u can always decrease the aperture such that u can see the object well lit but the background is darker, then increase the shutter speed till u can get the background to a more dark shade, if even after all this u are not achieving the needed darkness, decrease the iso sensitivity one step at a time and that should help
if u have enough lighting on ur tank, u can always decrease the aperture such that u can see the object well lit but the background is darker, then increase the shutter speed till u can get the background to a more dark shade, if even after all this u are not achieving the needed darkness, decrease the iso sensitivity one step at a time and that should help