tribaloverkill

First,... I don't know squat about electronics:( I just started working with LEDs. I made a couple of blue and UV case lights for my computer. I am happy to say that I havent burned any of them out yet:) I know how to figure out how much resistence I need for a string of LEDs. I have been figuring out new ways on being more efficient with my wiring too, that is using one resistor for the entire LED project as oppose to one for every three on 12V's. That the extent on what I know. Anyways. I have a little something I would like to do but dont know how to do it. I had asked in another forum some time ago but cant seem to find that forum. I want to power several LEDs inside my case but I want them to flicker along with the HDD activity light. I was told to use an opTo coupler and that was it. The only thing I know is that it contains an LED and a light sensitive detector thingy. A low power source power the LED and when the LED lights up the light sensitive thingy begins to leak a higher power source. Right? Anyways,... I dont know which one wot get, where to get it, how to wire it. I mean I have an idea but I doubt im even close. I want the opto coupler to be powered off the HDD jumper and then I want pass the current from my PSU through it to power my LEDs. I would also like a 3-way switch to switch from OFF - ON - HDD. I dont know what to do. Also,... I want to buy some rheostats to control boths my water pumps, which runs on 6 - 16V and needs atleast 18W. Also,... all the fans in my case. How do I know what rheostats to get? Is there a way to set a min and a max volt on them?