what does a skimmer do???

they remove disolved organics from the water column.

which basically means that they take out all the gunk and bad stuff that floats around and in the water, leaving more pure, clean, healthy water.
 
What they do is remove certain dissolved organic compounds as well as smaller particles, thus removing them completely from the water before they can break down (i.e., no byproducts to pollute the water). Along with that, they will often take other complexed compounds, such as copper, iodine, etc., which in most cases, is not a bad thing since they are already present in elevated amounts. The more refractory organics (terpenoids, etc.) that can be removed, the better, which is why I recommend activated carbon and good protein skimming.
 
well right now i dont have a sw tank. im just doing some reserch so i can see what i would need and about how much money i would need. how much do RO unit's cost???
 
Anywhere from $69 to a few hundred dollars. The ones I recommend generally run about $180 but as I asid there are less expensive ones.

The best way to approach saltwater tanks is to decide what you want to stock and then investigate the cost of an appropriate system. Make adjustments as you go along.
 
A good description of what a skimmer does was posted by Amphiprion.

A sump provides a place to put equipment such as the skimmer, heaters, etc that out of site. It also increases the water volume which increases the stability of the tank.
 
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