Anyone who has had less than crystal clear water knows that conventional filters have their limitations. When faced with stubborn particulate matter, or the dreaded scourge of green water, it's time to consider a diatomic filter. The name comes from the use of diatomaceous earth, which has the effect of polishing the water to perfect clarity. Here are my top picks.
Hands down, Vortex is the best. They've been on the market for decades, and have a well proven track record. Although the manufacturer states this filter can be used continuously, I advise against it. The same high filtering capabilities also remove other constituents that are desirable. Use only for short periods of time to polish the water.
The H.O.T package is not a diatomic filter in the truest sense of the word, because it does not utilize diatomaceous earth. However, the fine diatomic sleeve does remove most fine particles. It is easy to use, can be moved quickly from tank to tank, and has an optional cleaning attachment that is great for regular maintenance. For all around versatility, it's the best product you can buy.