Yes and what exactly does that have to do with over skimming? We're talking about two different things.

Thanks Denny that's a long read but very good

You said that skimming removes about as much bacteria as a water change, which really isn't the case. It sounded like you didn't understand how skimming worked.

:pop2:

A skimmer can only remove bacteria from the water column, same as just removing the water. Bacteria adheres to surfaces significantly more than the water itself thus removing the water or skimming heavier will not diminish a bacteria population period. This applies to carbon dosing or zeo, over skimming has no negative effects on a bacteria population, even if you're dosing it provided you dose downstream. So my point was when over sizing a skimmer to your bio-load the risk relates to removing excess essential elements such as potassium not bacteria. Make sense yet?

I understood your central point from the start, however saying a skimmer removes as much bacteria as a water change was mixed in there. Fact is, you can't compare the two. And truthfully a skimmer can remove a lot of bacteria.

Ok, but if you had one, it can help you determine whether your bioload is too high. :)

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.

Absolutely, found this out buy only running skimmer part time. My skimmer is rate more than double my tank volume and while running 24/7 I only ever got murky water and found it hard to tune the skimmer. Now running at night my skimmate has become very dry and sludgy.
This may put me at a light bioload for skimmer but fully stocked for tank size

Disagree, if for some reason I wanted to remove bacteria from the water column I could change 100% of the water which would be far more effective than what a skimmer could do. Ultimately changing a given amount of water on a certain frequency would compare directly to what a skimmer would accomplish, same as removing any organics. But that's all pointless information irrelevant to anything we're talking about here, the comparison was only made based on the common misconception large water changes are harmful because they remove beneficial bacteria, so I was making the same comment relating to large skimmers...