| Because sand and gravel filters are a bear to clean. There is almost no way possible in the application where you are putting pond water into sand, that 1 you are getting any filtration, 2 that you really ever get the crap out.
In my experience, any filter that is a bear to clean, or a real mess when you do clean it will only be cleaned maybe once or twice and then it will either be forgotten or the task relegated to someone like me who does it for a living. Problem is, I dont clean crap systems like that. I will help you design a system that you can and will clean, but you can not afford to have me come by and clean out a mess like that more than once. Believe me.
Then you start talking about the actual ability of the materiel to do filtration. There are thousands of items, many of them down right cheap, that are very easy and lightweight when it comes to cleaning. HEavy and without much surface area items never make a good filter materiel. Ever.
The only way I would ever use a sand filter is to polish already super filtered water. That means you have a settlement chamber, a fines removal filter (just like a bead filter, but with different media), biological filter, probably through another bead filter, then and only then a sand filter. Otherwise it channels way to bad to be of any use, and usually within minutes.
I have seen a huge sand filter with a 5 HP pump that was below the pond level set up this way. You could back wash the filter, and for about 90 seconds you would have a decent flow. Small for the HP of the pump, but decent. After the 90 seconds, it would keep getting smaller until after about 5 minutes the garden hose would carry more water than was passing through the filter back into the pond.
So no, for the vast majority of ponders, a sand filter only leads to poor water quality and poor performance both flow and filtration wise.
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