Comparing a skimmer and a bio filter is like comparing a car window and a car tire. Not very related. A bio filter converts chemicals...that's it. Many that you buy also have foam inside to collect muck which in theory would make them a mechanical filter, at least for a a couple of days before it clogs. A "combo" is of course neither. They're just marketing money makers.
I have seen few ponds that required a bio filter. Or I should say a retail bio filter. The pond itself acts as a bio filter. Bacteria actually have no idea they're sitting on some floating piece of something in the water, on the side of the pond or inside a $500 bio filter. Almost all ponds have low enough fish loads that no additional bio filter. The answer is always in the ammonia level. Zero level and no additional filtering is needed. And if you stop and think about it, what are the chances that that little green bio filter attached to the pond adds exactly the amount of filtering the pond requires? It's near enough zero I'd call it zero. If that filter is actually needed how long before even that isn't enough? Fish grow a bit, get fed a little extra and you're over the limit, ammonia builds, fish die. So it is always about measuring ammonia level. A bio filter does nothing to keep fish safe if ammonia isn't measure. That's assuming of course if you're worried about ammonia. Most ponds have such low fish loads ammonia is a non-issue and really doesn't need to measured at all.
Skimmers have nothing to do with bio-filtering (beyond adding a bit more surface area) and nothing to do clear water.
Skimmers are simple to make. Takes less time to make one than to order and install a manufactured model.
Here's my how to page.
Easy to make, almost free, easy to maintain...what's not to like? It's easier to remove a leaf when it's a leaf than when it has broken down into cells and chemicals.
Skimmers also act as the best pre-filter for pumps I've found, and I've tried many.
Skimmers' bread and butter is removing stuff blown into the pond but once in awhile they have another use. After you've had a pond for a few years you can get a build up of dissolved organic compounds (DOC). A normal waterfalls, not enclosed, will produce bubbles on the water's surface. Looks like bubble bath soap. Without a skimmer the foam will accumulate on the surface. A net can be used to remove foam but it'll just start building up again. As it collects in the skimmer the foam is easier to remove and the surface looks better. Poor man's protein skimmer.
Some kinds of algae will bloom on the surface. Kind of a green surface slick. Skimmers will screw up its life cycle.
Dust. Without a skimmer the water surface can look dull from dust on the surface. Surface tension keeps the dust from sinking so it just lays on the surface. Most people won't really notice except when the light is just right. But there is a difference you may come to notice after tending ponds for awhile.
