Reverse Osmosis is the best way available to prepare most residential drinking water in the United States for aquarium use. Water comes out free of 90 to 100 percent of all impurities. With so many configurations and options on the market, which one should you choose? There are several things you should consider. First, how much water will you need? Will you want a system hooked up to your sink? Do you want to set up automatic evaporation control? Do you want to drink the water? As a general rule, more, larger, and finer prefilters and residential membranes producing 50 gallons or less are best. If your PSI is less than 50, get a booster pump. You should only consider DI if you have problems with silica and/or phosphates, otherwise they tend to be overkill.