How the heck do I open this thing?

A nice simple question. I've had a prebuilt WCPC from Overclockers for a few months now. It's time to top up the reservoir, but I can't get it open. I wasn't supplied with any instructions or special tools on how to do this.

Reservoir is an EK, not sure what model.

pictures will be useful

If it's a tube rez just hold the tube and unscrew the top or take out the stop plug on the tube top, the inlet / outlet will at the bottom mostly.

Sorry for the delay, images:

http://imgur.com/Hv1bfoK
http://imgur.com/ztSSXer

Tried unscrewing the top but it won't unscrew, the whole thing just rotated within the housing so I stopped lest something awful happen.

Allen key to the top bolt, hold the reservoir whilst you unscrew!!

Hold the clear tube with one hand and twist the top section anti clockwise with the other hand.

Both black top and allen head bolt unscrew. Its probably just tight as to make sure it does not leak during shipping. Just hold clear part tight and unscrew.

i'm suprised no one has said don't just top it up with tap water, i used to use mayhems ultrapure H20 and biocide

My basket at Overclockers UK:

• 1 x Mayhems Ultra Pure H2O Watercooling Fluid 1L= £4.99
• 1 x Mayhems Biocide Extreme= £5.99