One of the lights on my small salt-water tank (11 gal) went out. Unfortunately the fluorescent bulbs that it uses are no longer available.
All the new lamps are LED (CREE) and cost about $150... I like my fish and all, but he was $10; and there is no way that I'm spending that much.

Since the old light was crappy, old, broken, and doesn't have bulbs available I decided to build a new lamp.

After a considerable amount of research I determined a few things:
1) Most Aquarium sites were recommending that I get a 12W CREE LED lighting system for a 10 to 20 gallon tank
2) Most Aquarium sites were recommending a 2:1 Blue:White ratio of LEDs
3) CREE lamps will kick out approximately 100 lumens per watt (YES, I know it varies, but I needed a ballpark number)

4) Therefore I need approximately 1200 lumens with a combination of Blue and White LEDs

NOTE: After preparing the lamp with 2:1 Blue:White, it was a little too blue/dark.... so I added 6 more white LEDs that helped a lot. Added ~377 lumens for a total of ~1800 lumens.

Step 1: Old System

Here is the crappy old light. It has a number of problems (other than being broken),
1) It leaves a pretty large gap above the water,
this allows a lot of splattered salt water to escape (and coat things with salt)
2) It runs relatively hot, causing evaporation
3) It is hard to get out of the way for cleaning purposes

Note the thermometer and water circulator in the back, and yes my tank needs to be cleaned :)

That said, with both lights running, I calculated it to kick out approx. 1000/1500 lumens, mostly directed into the water.

I wanted to use 5mm LEDs which have come a long way since I was a kid.
The flat-top LED's from C-LED seemed like a good option. This site gave me good service and quick turn-around... I was really appreciative. They kick out 20,000 mcd (white) and 13,000 mcd (blue) at ~120 degree angle.

So I designed an array of 20 Blue LEDs and 10 White LEDs for (816 + 628) = 1444 lumens. I decided to run this on a 12V power supply (wall wart).

Tool for LED arrays
Tool for Lumen calculation

Step 2: Plexiglass and Other Materials

I designed the lamp to fit entirely over the tank, but with a couple "tabs" to hook around the back while still leaving room for the temp controller and the circulator.

The LED's are inserted into the sheet of plexi. with holes. and that is placed inside a completed box. This allows me to pull out the entire LED array to work on it in the future, without needing to work inside the box, or take apart the box itself. This also protects the electrical components from the salt water (bad!)

I designed it with a removable lid, with a mirrored sheet of plexi. on the inside of the top so that I could get at the wiring if I needed to. This both reflects light down into the tank, and keeps the lid from sliding off.

Step 3: LED Wiring

The LEDs were placed in the holes of a sheet of plexiglass and the leads were bent over to connect them to each other. Then hot glued the LEDs in place. I did this before I soldered them together so that they wouldn't roll/slide around. Then soldered the LEDs together and the resistors into the array.

Last, I put together some wiring to connect the array to a power supply adaptor. The power adapter was hot glued into the side of the plexiglass (eventually).

The use of the adaptor was a nice feature because I can disconnect the power supply easily, and remove the lamp from the fish tank without needing to unplug the wall wart. It also means that I didn't need to modify the power supply (which feels a little safer to me)

Step 4: Build a Box

The lamp box was build out of "scrap" acrylic and plexiglass (1/4"). all the pieces were epoxyed together with epoxy designed for plastic/acrylic/plexiglass. It smells awful and started to eat at the mirroring that went on the lid, fortunately the mirror is to reflect light not so the fish can see himself.

The LED array is completely enclosed, including a the bottom that faces the water.

The lid was made of a sheet of plexiglass with a second sheet of mirrored plexi. attached.

Step 5: Finish and Total Costs

The finished light sits snug on the top of the tank and casts relatively little light on the wall around it. It removes easily from the tank so, its easy to get inside and clean.

It runs about 3W of total power and draws 240 mA.

Costs:
LEDs and resistors (~$20)
Wire, Power supply, 12V Connector (~$20)
Plexiglass (FREE, special thanks to my sister)
Epoxy (~ $10)