There are some good reasons for using pendant lights. When a light fixture sits right on top of the tank, the PAR near the top of the tank is always much higher than that near the substrate. So, even if you use a light which gives you very low light at the substrate, up near the top of the tank you have far higher light intensity, making it more difficult to avoid algae problems. When the light is located a foot or more above the top of the tank, the difference in PAR between the substrate and near top of the tank is far less.
Lights hanging high above the tank make in-tank maintenance much easier, and more likely to be done. And, when you are doing it you can easily see what you are doing. Also, pendant lights can look very good.
A major disadvantage of high hanging lights is the spillover light, and the glare. Those can be minimized or even avoided if a well designed pendant light is used.
My low light 65 gallon tank has a LED light sitting right on top, with the PAR at the substrate around 25 PAR, which is definitely low light, but the light shining on the Hamburger Mattenfilter, which extends up above the water surface, is high light, and, I have BBA growing on the filter foam, which has gradually migrated to the whole filter foam surface, making it easy for it to also migrate to the plants.
I want to try using 2 pendant lights, each centered over an 18" square of the 18 x 36 foot print of the tank, with the lights hanging 12 - 18" above the tank. Because I enjoy working with LEDs, I'm building my own lights, based on 10 watt LEDs from Ebay.
To minimize spillover and glare, the LEDs will be mounted inside cylindrical "cans" 4 inches in diameter and 6 inches long. I want the lights to be sturdy, not at all fragile, and I want to add low power red and blue LEDs to improve the color rendition of the cool white 10 watt LEDs.
I found some heatsinks made for 10 watt LEDs on Ebay:
For the "cans" I started out expecting to use 4" aluminum ducts, made for clothes dryer installations. But, then I found some 4" aluminum tube, with .085" wall thickness, on Ebay at a good price, so I chose to use that.
I bought a 12" long piece, and cut it into 4 smaller pieces, using a hacksaw:
I cut two 1/8" long rings, to use as retainers for the heatsink ends, then cut the remaining tube in half, to end up with about 5 7/8" long "cans".
I cut 17/32" pieces out of each of the 1/8" rings to make them fit inside the 4" tubes snugly.
After filing the ends of the rings and tubes to smooth them a bit, I used Loctite to glue the rings inside the ends of the tubes. They are now very tightly attached to the tubes. The heat sinks will fit against the rings, so the heatsink will support the tubes, with the "cans" hanging.
However, the heatsink mounting lugs, 4 places on each of them, are too big to fit into the cans, so I used a hacksaw and file to trim them back to fit into the tubes.
The heatsinks now fit in the tubes, and form the top of each "can".
Each heatsink will have a 10 watt cool white LED mounted in the center, with a blue and a red LED mounted on the sides of the 10 watt one. To expand the mounting surface to accept 3 LEDs, I attached a 1/8" x 1" x 2 1/2" piece of aluminum bar to the heatsink LED mounting surfaces.
The 10 watt LED will be held in place with 2 flathead screws, and the blue and red ones will be cemented in place.
Next will be to get a 10 foot piece of 1/2" electrical conduit, and make 2 hangers from it. To bend the conduit I found a used conduit bender on Ebay for about $12, so I bought it, figuring on relisting it on Ebay when I finish with it.