http://www.ro-man.com/shop/popup_image.php/pID/150
sorry to butt in on the post but i am having the same trouble too,
filters , carbon filters, i have not a clue which to replace, what are the 3 things above the filter housings called too lol. sorry but i got it second hand so no instructions.
and i cant find that spanner thing 
The three vertical canisters at the bottom of the unit are the Pre-Filter housings. These need replacing the most often, they typically are 5 micron sediment filter, Granular Activated Carbon filter and a Carbon Block filter.
They need replacing every 6 months as if left the material can start to deteriorate, if you use a lot of water then they would need replacing every 3 months.
The Horizontal chambers above them are the RO membrane housings. Inside each one is a reverse osmosis membrane. These generally last about 2-4 years, depending on how often you have changed your pre-filters and how much use they have had.
If production rate starts to slow then the first thing to do is change pre-filters and allow them to flush for a few hours. If after a few days the production rate has not returned to normal then the RO membranes may have been affected and need replacing.
Most RO suppliers sell the spanner thingies as well.