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.