bubble kings are no doubt good skimmers, but the alpha's have had a few issues (it's worth noting that they aren't truly "bubble king skimmers") IIRC most of the issue on the alpha's is that the diameter on the neck of the cone is enormous, making it very hard to get solid skimmate production unless your truly have a gigantic bioload... and also a fairly deep sump (about 10" of water is recommended i think).
I spent month's debating over this, and ultimately went with the ATB 840 v1.5 over the alpha 170 or 200.. it was a bit cheaper, likely performs a smidge better on slightly smaller tanks (below 150 gallons, upwards of 150 gallons the alpha would likely have the upper hand) and it's still a fantastically built and designed skimmer. Only issue i have is that the ATB mini-airstar (a modded sicce) isn't quite as nice as the mini-red dragon pumps (which are truly 100% silent).... as far as performance between the two pumps however.. there were a few issues with the modded mini-red dragons because they are throttled back so drastically...
ultimately you can't go wrong with either skimmer, the Alpha cones, or the ATB cones, but my recommendation would be this:
On a tank smaller than 150 gallons, or if you don't want your sumps water level to be 10", then get the ATB 840, and even save yourself a bit of money in the process.
On a tank larger than 150 gallons, and if you don't mind the extra sump depth, get yourself the alpha cone skimmer, with a quieter red dragon pump =).
My tank fell into the first category, so i got the ATB 840, which is an awesome skimmer just the same... FWIW though, the alpha 250 would be WAY to big for a 125 gallon system.. so if you don't plan on upgrading i'd look at smaller alpha models (the 170 or 200) as well as the ATB models (ATB 840 v1.5, or ATB 840 Deluxe)
