okay i tryed out 3.....20 watt coralife sun and reef 50/50 on my 20 gallon long tank which is only 10 inch,s deep .......it seem to work alright...............i have coco dusters feather dusters,sponges.1 small clam, fire scallop and one with ridges........i also have dif polps,daisy polps,red...