The tank is only 12 inches deep so 4 would be ok, but with a 24 inch width I would probably go with 6 bulbs to get a better coverage. I have a 5 bulb on a 45g cube and it covers it, but I kinda want a six bulb or bigger. Just my opinion, again if you are just going to keep zoas/palys the 4 bulbs will be sufficient.