The one in the Koiphen plans is quite large. I downsized it considerably in 3 versions.
The first Koiphen DIY I made (in my video) was about 4 ft tall X 4" in diameter.
I later made one half as tall and 3" in diameter that ran from an overflow of a 150 gal tank, into a sump below.
I then tried one 8" in diameter by about 3ft tall.
I don't remember exactly what the cost was, but PVC was no more than $30.
The ones made from "clear" PVC brought the price up because clear is about $20 per ft.
But clear PVC is not a necessity, I used it out of curiosity.
It is the water cascading over the lava rock the breaks (fractures) the air water interface and separates the pollutants.
I found the fractionation takes out proteins, free floating algae, and even some microbes.
I studied skimmer waste under the microscope and found a @10 to 1 increase of ciliates, nematodes, and other microscopic life compared to normal, straight tank water
I also used it on a kiddy pool I kept Tilapia in, catching the waste foam in a bucket, using zip ties to lash it to the fence.
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Do you feel as if the skimmers along with some sort of freshwater nitrate reducer would allow more time between water changes, or is it really more of a "keep the water quality as good as it can be between a regular amount of water changes?"
I ask because I have heard of some saltwater guys not doing any water changes because their skimmer pulls out all of the TDS and then macro algae, deep sand bed, etc, would keep their nitrates down. They then dose necessary minerals to keep the fish/corals/inverts healthy.