They do it for space saving but moving a pump out of the body does not mean a better skimmer. A skimmer has to be well designed to begin with.. My RLSS skimmer will out perform allot of skimmers with external pumps. Open volute skimmers can perform nearly as good as skimmers with external pumps. Really comes down to design. A external pump does increase contact time and should out perform but does not always.
I am going To say this too about the comment about the Aquamaxx priced cheaper than the same rated ATB. Skimmer ratings mean nothing and the ATB is far closer to its actual rating than a Aquamaxx. Almost all skimmer can not handle what they say even at normal loads.
Another example the RLSS R6-I is rated for up to 150 gallons.. The coneS Co-1 is rated at 175 gallons. The RLSS pulls 960 l/hr and the ConeS 360 l/Hr. Both are well designed skimmer but really the coneS is rated more than the RLSS skimmer.. I own both and not even a comparison. The CO-3 is rated 500 gallons, come on. I guess it could be very little load. My point is skimmer rating mean jack.
You need to look at the design of the skimmer, the air pull, size of the neck, size of the body, type of pump, the height of the skimmer, where the pump is located etc.
Size of tank means nothing either. I have seen tank at 75 gallons with more load than 150 gallons.
I completely agree with you. I played the skimmer game for a while... I started with a turboflotor (back when AM only had one model), then ETSS, then Euroreef, then a break from reefing, then ATB and Vertex Omega. Through each generation of skimmer I saw an increase in efficiency, better designs that offered better air/water mixing, better pumps, and better construction. I also saw the rise of knock-offs and crappy skimmers where ratings became arbitrary.
I have a hard time believing that the short skimmers I've been seeing with a pump mounted inside -- even with unique features like bubble plate with custom diffusers-- will perform better than a similar skimmer with an externally mounted pump where the water enters lower in the reaction chamber. For example, the Aquamaxx ConeS Q-series versus the regular ConeS. Originally I liked the Q-series because it was shorter. However you can quickly assume that the ConeS is a better skimmer:
ConesS CO-3:
22.9" x 8" reaction chamber at 1020 LPH (rated up to 450 gallons @ 27 watts) - 22.9" is actually the total height of the skimmer so the reaction chamber is probably 21"
ConesS Q-3:
19.7" x 8" reaction chamber at ??? LPH (rated at 420 gallons @ 27 watts) - the problem with this is the total height is 19.7" and the pump is on the inside, which technically means the reaction chamber is really only about 15"
Both use the Shark 3.0 pump so we can assume that the LPH for the Q-3 is the same, but the reaction chamber is smaller.
Moving on to the ATB:
19" x 8" reaction chamber at 1200 LPH (rated at 260 gallons @ 16 watts)
Of course, the ATB is a few hundred dollars more. This
is taking into consideration ratings given by the manufacturers and I tend to believe ATB's ratings as you mentioned.
Yes, there is a lot to take into consideration, but it's really hard to compare skimmers based on what's on paper. My gut tells me to go with an ATB because the one I have has been rock-solid.