help with choosing the right skimmer

total water volume minus all your rocks, sand equipment etc as stated above. I would get the best you can afford. I would get one that was for a higher water volume lets say you needed 150g I would get a skimmer rated for 200 - 250. This would give you some the ability to upgrade some without buying another skimmer, the skimmer will work more efficient rather than trying to drive the skimmer to it's maximum potential.
I have read on many forums from people that have used and tested many skimmers that some manufacturers over rate their skimmers. There is exception of a few that seem to under rate their skimmers. So I would buy a brand that is a well known brand that has good to great reviews. I like the conico cone skimmers that have been produced in the last few years. If things start to turn around I will be buying one and trying it out they are supposed to be very good skimmers.

Bill