The trouble with most ceiling is the josts never seem to be in the right place.
I'm not sure of your DIY skill set. Lets start with position. Get you tank where you want it first. Take a string with a weight like a steel washer tied to one end. Hold the string to the ceiling above the tank with the washer dangling down. Position the washer the spot were you think you want one end of the light fixture. then mark the spot on the ceiling where you are hold the string with a PENCIL...never use ink on a painted wall, repeat for the other side.
I like to keep my lights a bit behind the center line of the tank. Most of my planting is there vs. the front glass and I think it's easier on the eyes to view.
Once you figure where they go you need to know whats behind that drywall. The easiest way is to use a 3" finish nail and a hammer. Gently hammer the nail in the drywall, most dry wall ceiling will be 1/2" if the there is a joist behind the dry wall you'll feel it after the nail is 1/2" in.
The joist will be apps 1 1/2" wide if there is a joist right over the tank, them you can use an eye hook and screw it right into the joist. tThat will be very strong. If not you can use the butterfly anchor.
A stronger approach would be to find two joists and attach a 1"x3" painted board to them (Think track light, trank). Then you can eyebolt two the board. this is nice if the joist don't line up perfecty for your tank lay out. As long as you get two screws into the two joists it doest matter how the board is attached, it could be 8" over on joist and 12" over the other. BTW most joists are 16" center, center of the 1 1/2" board is 16" to the center of the next one.