With such a large selection of RO and RO/DI systems available, it is often difficult to decipher what a hobbyist or an aquarium store needs. Here is how we help our customers pick out what they need. The biggest and most-expensive system is not always the best for every scenario.
1. RO (reverse osmosis) or RO/DI (reverse osmosis / deionization) - If you are setting up a coral reef aquarium, you definitely want the purest water possible. An RO/DI system is the one you want and need.
If you are setting up a saltwater fish-only aquarium, a brackish aquarium, a freshwater aquarium, a planted aquarium or a crystal shrimp aquarium, you will still want clean and purified water. However, 100% pure, zero tds, water is not required, so a RO system is more than sufficient. An RO system will remove approximately 95% of all the impurities from the water and works great for these applications. 
2. Chlorine or Chloramines - all of our systems will remove chlorine efficiently and effectively. However, the standard carbon filters in the RO and RO/DI systems will not remove chloramines. Chloramines is a combination of chlorine and ammonia that is great for water sanitation, but horrible for aquariums. As more and more municipalities are starting to use chloramines for water treatment, you will want to find out if your water is treated with chloramines then choose one of our systems with the Chloramine Blaster Upgrade if it is present in your water supply.
See our Chloramines article for more information!
3. Size (compact, full-size, light-commercial or larger) - we have RO and RO/DI systems that are suitable for any size aquarium or aquariums. If you have just a nano reef or a smaller aquarium less than 50 gallons, our Seahorse RO and RO/DI systems are a great match! They are compact and super affordable.
*please do note that the Seahorse RO and RO/DI systems cannot process chloramines. If you water contains chloramine, you will want to upgrade to one of our full-size systems with the Chloramine Blaster upgrade.
For medium and large size aquariums up to 1000 gallons, you will want a full-sized system. About 80% of our customers fall under this category. The Barracuda Glacial RO/DI is our #1 selling system for a very good reason. It will give you ultra-pure water at a good speed and at a great price!
For the budget-conscious, our Barebones system is great for those that want ultra-pure water without the bells and whistles.
For those that want the top-of-the-line, our Afishionado-Special systems have all the desirable gadgets and the extra filtration stages to give you the cleanest water possible.