In the natural world, objects frequently have more than one purpose. For example, a Damselfish comes to a sudden stop in the water with its pelvic fins flared downward which makes it easier for the fish to slow. A Goby comes to rest on the bottom and the shape of its pelvic fins enable it to cling to a rock more securely. The pelvic fins in both examples are homologous structures but they clearly perform separate functions. Similarly, in the equipment-filled world of the aquarium hobby, the reverse osmosis  (RO) filter is a tool that is also capable of performing multiple functions. The intent of this article is to provide further clarification on several of them. A reverse osmosis filter is undoubtedly a useful piece of equipment to own, but before you purchase one you first need to decide what you need it to do. Most aquarium hobbyists have seen ads for RO filters online, in magazines or in catalogs. But what did they take away from viewing such ads other than prices and promises of purer water and a healthier aquarium? The expense alone of buying an RO filter demands that you do your homework before purchasing one. The fact is that not all filters are equal and ads alone are just too little information to go on. You need to decide which RO filter is correct for your particular application. The general design of most RO filters is basically the same but the filter cartridges and membranes that come with the various models di ...

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