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I did Google and CF search this, but didn't see any uses for sand other than substrate. So...

I was thinking about ways to improve the efficiency and reduce the cost of filtering aquariums and I got to thinking. Can I use pool filter sand, not as a substrate, but as filter media? I mean, that is what PFS is for after all; filtering swimming pools. It seems to do a pretty good job at that too, so...why not aquariums? Undergravel filtration has been used for years, and obviously works. But...

What about a sump, putting intake/return aside, just thinking about filtration for now. Two compartments, divided by a baffle 1" off the bottom. PFS filled in both compartments for 3-4", or higher. Theoretically, water would flow down through the sand from one compartment to the other, trapping all mechanical waste on top of the sand. Cleaning would consist of vacuuming the sand for detritus. Reusable filter media. It would allow you to employ a large, reusable, cost-effective, undergravel sump.

A few questions come to mind though. Would the water flow freely through the sand or cause the sump to overflow? Would an enclosed tank or container be required to force the water through the sand with a pump? Briefly reading how pool filters work, the water is gravity fed through the sand and flows back up, or something like that, to get back to the pool. But its gravity fed, giving me hope. The filter media in pools is then cleaned through the process of 'backwashing' the system. In other words, reversing the flow of water and draining it before it returns to the system. Vacuuming the sand should accomplish much of the same.

I'm sure this has been done before but I couldn't find anything on it, so please be kind if I'm late to the party.
 

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There is no doubt that filtering over sand can be done, but personally I think the disadvantages will far outweigh any advantage sand filtering might offer over any of the filtering systems I am using. My hunch is that the main difference between filtering pools and aquariums is the amount of debris that needs to be filtered out. Large fish, like a lot of cichlids, produce phenomenal amounts of waste, that would quickly clog up the tiny space between the sand particles. In a pool you have very little debris in comparison, and the tiny spaces between the sand particles are probably a plus because you will catch absolutely everything that goes through the filter.

I would foresee the following problems in using sand to filter fish tanks:

1. The flow rate through the sand will be super slow. The only way to avoid this is to bring the sand into a fluidized state, as done in Fluidized Bed Filters (FBF). FBFs offer a fantastically large surface area - so large that they are overkill for any aquarium application I can think of. In addition, FBFs are a pain to clean when the need arises. See here for my FBF experiences.

2. The space between sand particles is tiny. A thin layer of debris on top of the sand could easily block the whole filter, making frequent maintenance a must.

3. Cleaning the sand by vacuuming it might sound easy enough, but is actually quite time consuming in practice. I far prefer to wash out my sponge filters or the filter foam mats in my sumps. In my canister filters I use filter floss - actually quilt batting from Walmart. The stuff is so cheap that I just throw it out, and in practice there are enough nitrifying bacteria in the tank itself that I observe no ammonia spike when doing so.
 

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In a pool filter the water goes through the sand under pressure, in an enclosed vessel.
That`s what keeps it in suspension and suspension is why it flows.
Maybe a series of thin sand layers would not cause your idea to wet the floors right away, but you sure would need to monitor that thing constantly.
These filters(sand) are used in huge aquaria, both salt and fresh.
In those systems, they are run just like a pool.
Huge volumes of water, under pressure to keep the sand in suspension.
 
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