My .02 on skimmers. There is so much to take in when you look at skimmers. Design, size, neck diameter, transition to neck, bubble plate, input from pump, standpipe....etc and thats not even looking at the pump. The transitional cylinder NW skimmer is an easy skimmer to talk about. Look at the ER's, Deltec's, Octo's from a few yrs ago. They were great skimmers for there time. They were pretty simple designs as well. A decent design, nice size body and decent pump (some anyhow) Now the newer skimmer designs all have bubble plates and most like the msx/octo extreme have enlarged neck's and better body to neck transitions, almost like cone's. Now the design has gone one step further and made a cone skimmer. Easily the smoothest design out there. Just for the simple fact there's no transition, less backpressure on pump's so they can work like an unrestricted pump.
Now the ratings are different between the cone's and traditional cylinders. I generally run a 4-6-8lph air per gallon of water for a well designed skimmer. So a skimmer pulling 600lph air is good for a 100g tank with a medium bioload. Now when it comes to the cone's I'd go with a 1.5x's higher rating so a 600lph air is good for a 150g tank.
For example for air pull cylinder vs cone. The octo pro 300 pulls 70scfh of air or roughly 2000lph air and 600gph waterflow. Pretty unbelievable imo. I'd rate this at a good 333g tank with a medium bioload. But i'd rank this pump on a cone skimmer at 500g on a med bioload.
The pump's out there right now that are some of the best are the sicce psk2500 as its the cheapest pump that will pull from 900lph -1400lph air and only pull 350-450gph water flow at 17-30w. WM has done a great thing with the sedra 9000's giving them around 1100-1200lph air at around 40-50w. The airstar, laguna maxflo 1500, Red dragon, askoll 1500 are all the same motor block and all pull 50-80scfh or 1400-2266lph air at around 50-60w. The octo pro 300 i'm running is pulling 75scfh at 57w hooked up to the skimmer. The askoll pump's are THE BEST pump's out imo.
Now for the impeller's and bubble size. There's three different impeller's out, the NW-needlwheel, pw-pinwheel and mw-meshwheel. And they break down the bubbles smaller and smaller. The nw being the least effective and meshwheel making the air bubbles teh smallest. I prefer the PW as there's no maintenece required and your getting very fine bubble size and don't have to worry about any start up probs like you do with mesh. The smaller the bubble size, the more surface area the bubbles have to bond with doc's to produce skimmate.
Dwell time......well this is the toughest thing to challenge imo as far as skimming goes. The recirculating vs in sump skimmer challenge. Well, with the newer pump's that pull so much air and so little water are the recirculating skimmers that much more effective? WIth the older skimmers using genx4100's and otp 3000's sure using a feed pump of 300gph vs 1000gph water flow it makes since. But now with a sicce pushing only 350-450gph or an askoll pushing 600gph do you really need to have a recirc? Maybe if you had 2-3 sicce's or two askoll's it would benifit you. As for dwell time, thats very controversial. It use to be a 2min dwell time. Not sure where this # came from, just kinda spawned out of no where imo. Dwell time is important but not so much as were looking to recycle this water over and over. Push the water through and give it some contact time with the air and push it out and bring in some new water. No since in keeping it in the reaction chamber to long as the water can only be stripped so much.
Pretty generic skimmer thoughts but I will be happy to explain anything further if you would like.