can you send me some photos of this power supply mod you did? i work with computers and am about to put two finnex planted plus's on a 75 but would be willing to take a risk and see with a old pc power supply if i can add a dimmer inline for my 33long in my signature.
I don't know how to post a pointer to a specific post but look at post #114 in this thread:
For the LED+ all I am providing is 12V. This isn't a dimmer -- I use a IR transmitter to let the LED+ control the dimming. A PC power supply by its nature has +12 available. What's needed is to pull it apart very carefully and you'll find that some wires may be wired together -- keep track of those and keep them together, and cut the rest. One wire (usually green) will need to be connected to ground (usually black). Google for ATX power supply and there's all sorts of wiring diagrams around. The thing you want is to make sure that your power supply has a SINGLE 12V section with significantly more wattage than you need. That's usually easy but it depends on your wattage. Note some power supplies might be 800 watts but in 3 sections, but somewhere on the specs it will give you watts in each from each voltage.
If you are building your own dimmer for LED's, do NOT use a resistive dimmer as it is very wasteful. Get something that will do pulse wave modulation (e.g. it is pulsing fast, and the percentage of the pulse that is on vs off determines how bright). Look for "PWM" if you search. You can build your own with something like a Raspberry Pi or Arduino and a few transistors. The nice thing is the PC ATX power supply (besides being able to recycle what would otherwise be trash) is it can also supply +5 or +3.3 as needed to the cards. Mine powers 4 LED+'s, one Raspberry Pi, and has a separate 12V I use for a bilge pump to do transfers from a trashcan into the tank (i.e. for water changes).
Getting 12V from a power supply for a self contained light (i.e. built in controls) is really easy, you don't even need to solder, you can just crimp connectors. Building a dimming circuit is a lot more complicated.