You can use a fan controller on the 900 with some limitations
1. You need some kind of 3 pin to molex Adapter(a backwards one since you are not running a fan on a molex, you are telling a 3 pin fan controller to be a molex) since the Tricool fans only us the 4 pin molix.
2. Getting a fan controller with RPM monitoring is unless with the fans in the 900 as they do not send any kind of speed signal.
So? What to do?
First off get a controller with a voltage variance of 5 to 12 volts(this is to stop you from shutting the fans off)
Next if you can not get a reversed 3 to 4 pin adapter you can just use 3 pin extension cable cut off the 3 pin male end
the one on the right is the make end(for obvious reasons
)
Now cut off the molex from the fan and you will have 2 wires(Red = Positive , Black = Negative)
Now skin the wires and connect(twist, and solder if you can, if not just be careful not to pull then apart in the future) red to red and black to black....tape(electrical tape) up the yellow wire as there is nowhere to connect it to, but you do not want it loose. carefully tape up your wires in a way so they do not touch. first electrical tape the red one, then around the black one then tape them together(make sure they are well covered before this.). Last tape the yellow wire to them to keep it out of the way....
Now you can plug the tricool fan into any fan controller(3 pin) and vary it's speed.
Note:
Some fan controllers(zalman MFC 1) will not light up without a speed signal. It will control the speed but you will get no pretty light.
There may be a fan controller on the market that lets you use 4 pin molex right away. This would be a good option as it avoids any need to mod the fans.
due to the 900s open design i have found there to be little temp difference from low to med and almost none med to high.....so maybe you could just try low and see how it fairs....
EDIT
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Some Vantec fan controllers come with a reverse adapter....may be worth a look