I'm new to the forum, but wanted to share my experience with a DIY LED kit that i bought from RapidLED for my 20 gallon tank.
After lots of research I decided on the RapidLED 12 Ultra Premium LED DIY Kit with Dimmable Drivers 95$
For kit choices, I went with PWM dimming, white lens colour, 80 degree, thermal adhesive, and an LED ratio of 4 cool white, and 8 neutral white.
My choice in LED ratio was a strongly based on http://www.plantedtank.net/forums/ta...ope-w-led.html
as I wanted a tank that would both look good for fish (neutral white) and grow plants (cool white).
For a heat sink, I also went with RapidLED to save on shipping and ended up with Drilled and Tapped 4.25" x 23" Heat Sink for 24 LEDs (50$)
Because the system is only running 12 LEDs, even though its drilled and tapped for 24, I can run it at full power, for 12 hours a day without it overheating. I currently run at about 40% power for 6-8 hours and it barely gets warm. I don't think I would have a problem running it 24 hours a day at full power. The over sized heat sink just has too much surface area. I went with this heat sink because I wanted something that would span the length of my tank, and wouldn't require fans for cooling.
All included, with shipping I was out 180$ for everything.
In addition to these parts, all I took from my personal inventory was some heat shrink tubing, a coat hanger, thermal adhesive and solder. I vary the dimmable driver using an arduino mega 2560, but that is primarily used for my DIY PH controller which is another article entirely.
During assembly, I did at least one thing that I haven't seen discussed in the forums, that ended up working out in my favor. This was applying lenses, they help, a lot! Without the 80 degree LED lenses, the light off the side of the light fixture was blinding. After looking at the initial stages of the energized LED build, while it was mounted on my tank, I would be seeing yellow and blue dots for minutes after looking away. This annoyance was fixed after adhering the 80 degree lenses, but this would also be solved by creating an entire hood. I would recommend to anyone following this style of build to adhere lenses, be it 30, 40 or 80 degrees.
I also found the lenses help reduce the 'spotlight' effect which allowed for full covering, even considering the fixture only sits 2-3 inches off the surface of the water.
To mount it on my tank, I designed and bent a couple of coat hangers to hold it. I drilled into the side of the heat sink, and friction fit the coat hangers in place. They are pretty tough to get out. I plan on painting them or getting them powder coated fairly soon, otherwise the coat hangers will rust over time.
Bottom side, you can see I also adhered a waterproof LED strip that I purchased off Ebay. This is a RGB LED strip so I can add, red green or blue colour to the water column. I'm planning on using blue for moon lighting, and red when introducing new fish. The LED strip is very low in lumens and is only good for low light applications. I made sure to solder wires over the LED strip, just in case it loses its stickiness, there is no way it can fall into the tank.
I have gotten a lot of great information from this forum and I just figured it was time to start contributing. I hope this helps others with their own DIY LED build.