Will be putting together a new led light for my tank. I decided to use acrylic as the case and was wondering if 3/16" acrylic would be thick enough to use.

Acrylic, especially thin acrylic, will bow over time when used as a lid because of the heat from the light on one side. The way I've gotten around that problem was to glue straight strips of acrylic, on edge, around the edges on all four sides of the lid. use strips that are 1/2" or 3/4" wide, and glue them onto the bottom of the lid. The picture below shows what I'm talking about. You can either have the little "walls pointing up, as shown, or turn the lid over so that strips aren't visible (as long as they aren't so wide that they touch the water). The strips will keep the lid from bowing, and 3/16" will be fine.

I think we need a little clarification before you can get a good answer. You want to use it as a housing for a light fixture? Or as a lid for a tank? If it's a housing, 3/16" might be thick enough, depends on the size of the housing. For example, if it is 10" long you might be OK. 36" long and 12" wide... no way. You get the point.
Also depends on the heat generated. Heat is not a friend of acrylic. Will it have fans or will the heat just build up inside?

Chris

My bad. I misunderstood the question, and I thought you were making a tank lid. never mind.

Thanks for the replies and sorry for any confusion. I am makin a LED fixture with 2 13" aluminum square tubes and will be encompassing in acrylic. There will be a fan on one end. The dimensions will be ~ 15"x5"x5". See below pict, left off one side and the clear shield for easier viewing. Thnks again.

Should be fine, the aluminum tubes should hold it pretty rigid in the long dimension, and the endcaps of the housing will reinforce the ends rather well. About the only part that MIGHT deform is the bottom edges (as mounted over the tank) of the front/rear panels of the housing, but only being 15" I doubt you'd see any deformities. go for it.

Fan on one end, I assume that the other end has a grill or other opening to allow air to flow? If not and you just have the bottom open (bad idea, exposed wiring/electronics), your LEDs may not be cooled well enough.