Water Deionizer

[OscarFan]

What the heck is a water deionizer I saw one at petco for $17 when its normaly 60.

[benjie_eb]

Deionizers remove all Dissolved Solids and gases from water, producing water of top quality and purity.
Sodium, calcium, and magnesium (positive charged ions)
Sulfates, alkalinity, chlorides, carbon dioxide, and silica (negative charged ions).
End result no minerals, ions or salt in the water comming out of deionizer.

Can only tell you what they do.
As for the benifit in a aquarium i am not helpfull with maybe someone else can elaborate on that.

[Alfcea]

Unless your tap water is extremely hard or has a lot of dissolved salts, minerals, etc, and you are having problems because of that (like algae blooms), I do not think they are a very good idea. Dissolved minerals are actually good. Very pure water with no electrolites and a very low ionic strenght would be stressful for the fish, plants and bacteria since the ions are used for many biochemical processes.