Ah yes, but will the sheet rock hold it?LOL I've seen where the sheet rock has been pulled out by the weight with the anchors still attached.
Some people here have made hanging trees, see if you can find those threads, or if you're really handy you can probably come up with one on your own. Pipes, connectors (such as elbows & T's), solder, chain & hooks I imagine are the supplies you'd need.
Most manufacturers make hanging kits for their fixtures. I would not trust the weight of a fixture (especially since you said it is heavy) to toggle bolts on sheetrock. If you locate the ceiling beams then drill into them or like Awibrandy said try a hanging tree. Prefab is expensive but I saw a thread on MR for a DYI project. Good Luck.
What I did could work for you if you have a similar tank setup. I have a steel stand for my 55 and my fixture was too heavy and large to hand grom the ceiling. I drilled two holes on each side of the stand. A hole at the top and the bottom. Then I purchased steel rods that I cut to run from the top of the steel stand to 12 inches above the tank. I mounted the rods, one on each side, by putting screws into the holes. Then I drilled a hole at the top of each rod . I put anotehr rod going perpindicular to form an L. I drilled two holes at each side of the L and through my fixture and put the screws in. Now my fixture hangs over the tank.
they make c chanel metal brackets that you would attach to your ceiling with small lags to the main beams..either 24 or 16 on center.. then in the c chanel are rolling sliders with attachments for a threaded rod that you can attach to your light.. once you get it centered where u need it to be you can lock down the rollers....