filter?

I claim to know nothing on many occasions and here again I know nothing about this subject only what I've read today because my interest was sparked by Robs post. But I feel like there has to be a reason a ton of large aquariums use these filters? Weather is only benefits a large system Idk?

Really? So my sump which is one of my biological filter polishes my water? I don't know how as all that is in there is a deep sand bed and live rock for the bacteria to colonize on, That is a bacteria filter. A sponge, sock filter, skimmer, etc. is a mechanical filter. My anaerobic area has no water polishing capabilities at all. Now the filter sock after my overflow and skimmer those are serviceable water polishing mechanical filters. Yes there is a difference.

are you serious? What is the reason for bacteria? Answer to polish water the bacteria eat the organics polishing the water the problem is what is left is hydrogen sulfide (swamp gas)

Your talking in circles now.

You have serviceable filters. They catch crud and you clean it from the filter end of story..

So my sump is a serviceable filter, on a weekly basis I change out the live rock and sand. I guess I should change out the clean up crew and bacteria as well. Talking about talking in circles and using wrong phrases. "bacteria polishes water".

Depending on ones perspective, you could say that both types of filtration, mechanical and biological filtration technically polish the water by removing or reducing solid and dissolved organic or inorganic matter thereby leaving the water clearer and cleaner. While the process and maintenance may differ between each general method, the end result should be the same. Anyone's personal biased towards either side of this subject is completely depended on each persons interpretation and understanding of the definition of water polishing though.

I will say I'm skeptical as to any long term success with a flow through sand type pool filter being suitable for a reef environment. I would think the possible problems/maintenance required to keep such a setup clog free and working properly might outweigh any possible benefits it could have. That's just my standpoint though and I could easily be proven wrong,

Borrowed from Wikipedia

"The term water polishing can refer to any process that removes small (usually microscopic) particulate material, or removes very low concentrations of dissolved material from water. The process and its meaning vary from setting to setting"

It's a pretty vague definition but technically doesn't this mean that even a protein skimmer is a water polisher. In fact its not a vary large step to say that technically cheato or any living organism that removes or converts material from out of the water column could be considered a water polisher. Food for thought. Like I said it all depends on ones understanding of the term "water polishing"

There are 3 types of filtration: mechanical, chemical, and biological. Actually its a little more complicated than that. Let's explore...

Mechanical: filtration that mechanically traps debris in a media like sponge, floss, gravel, sand, etc. The key thing here is that mechanical filtration removes nothing from the water...it's just traps it. The point of mechanical filtration is the following:

• Polish the water (clarity)
• Concentrate debris for easy removal
• Protect subsequent filtration stages (like chemical adsorbents) from being clogged with mechanical debris

Chemical
: filtration that utilizes a medium's ability to interact (through adsorption, surfactant attraction to air bubbles, etc.) with undesirable molecules in the water. Chemical filtration does remove pollutants from the water, sequestering them in some way (carbon adsorption). Some chemical filtration acts on a gradient...under some conditions it holds on to the pollutant, but under others it will release it back into the water (in freshwater zeolite will lock up NH4 but in saltwater NH4 is released...that's how you recharge it).

Biological: filtration that uses a living organism to a) convert pollutants to less toxic substances (bacteria in the nitrogen cycle) or b) sequester pollutants (algae converting phosphates, nitrogenous waste, etc. into plant material). Biological filtration permanently removes (assuming your algae doesn't die off) or alters pollutants (NH4-->NO2-->NO3).

One thing to keep in mind is that any mechanical filter media and most chemical filter media will become a biological filter. This is because bacteria will opportunistically grow wherever they find a surface area and a food source. And mechanical filters (and chemical media like carbon) provide wonderful food sources w/ high surface area. This is why they should be sized correctly and cleaned frequently. You can crash a tank by cleaning your mechanical filter too well. This was a common problem back in the day when people who used under gravel filters/power filters would wash their gravel/foam in tap water, killing off all their nitrifying bacteria in the process. "But my tank is soooo clean now!"

So back to this sand filter...assuming it's saltwater safe...

• The purpose/design is as a mechanical filter for polishing large volumes of water that are prone to collecting lots of debris. Think swimming pools or aquariums with large fish that poop a lot (arrowana anyone?).
  
• It will need to be maintained by back-flushing. I have no idea how much water that takes, but salt isn't free. I personally think you'd do just as well or better for clarity with filter socks.
  
• (Repeating myself...) these filters are designed with a certain grain size in mind for the media. I am skeptical that you can find a calcareous media of the same size that will also not degrade to smaller grain size through physical grinding...calcium carbonate is softer than quartz, period. Even if you could find a calcareous media, I don't see the benefit as most calcareous minerals like dolomite are not soluble in the pH we maintain our tanks at. That's why our sand and rock remains sand and rock. You could use silica sand, as designed, but I think the jury is still on whether using silica sand in a reef tank is good idea. I'm guessing you could find a ceramic media...