Let's talk about bio load.

[Samw]

I think it does address the question. Your question was how do you know if your tank has light, med, or high bioload: "How do we measure that? I mean, quantitatively, what's a medium bio load? Or heavy? Or medium light? "

So with a DO meter, measure your DO first thing in the morning before there is any light when DO is at its lowest levels. If DO is at 50% or less, you have a high bioload. If DO is 50-75%, you have medium bioload. If DO is 75%+, you have light bioload.

$150 is cheaper than many protein skimmers. Or rent one from a lab: http://www.google.ca/url?sa=t&rct=j&...2hTWDbiE64d4zQ

Originally Posted by Aquattro

Ok, I'll give you that

However, that doesn't really address the question.

Skimmer x is good for 200g @ light load, 150g @ med load, 100g high load. How does one determine if skimmer x is appropriate for their 120g tank?

Personally, I've been doing this a couple years now, I just know. I know my bio load, pretty comfortable guessing bio load capacity, and I'm confident in my abilities to size a skimmer.
But for most consumers new to the hobby, what does that mean? Does skimmer x meet my needs? Or do I need x +/-1 for my tank? Generally bigger is better, but if I go too large, I may not have enough load to get proper foam development.

All theoretical discussion with no real answer, just something to kick around.

[Samw]

If your question was how to interpret a manufacturer's recommendation as to whether the skimmer is good enough for your bioload (once you've measured what it is), then nothing can of course resolve that since that is the manufacturer's recommendation. That is just up to the customer to trust the manufacturer and is simply a trust issue.