The basic process:
Essentially, an RO/DI system treats water in several stages. Pressurized water is forced through a course particulate filter, then the water is forced through a reverse osmosis membrane, which is a membrane with extremely small pores, smaller than most molecules. Water that actually passes through the membrane is purified (about 1/3 total volume). The rest of the water does not pass through, and carries away the salts, etc. that have not passed through. At this point, remaining salts are removed through the deionization stage. Some systems have additional steps.
I picked up a system meant for drinking water off ebay for cheap, that works very well.
Most people on this site have reef tanks, i.e., coral. Coral cannot form its exoskeleton in the presence of excess phosphate. Furthermore, most reef inhabitants are not as tolerant of metals, pollutants, etc. as fish are. So, I don't know if you really need RO/DI water for seahorses. If you choose not to use RO/DI, be aware that many water treatment plants add chloramine to water, which won't simply evaporate away.
If you do a search for "do I need DI water for seahorses", results come up. I don't know if it's frowned upon to give links to the other major reef site, so I won't. You can find what you need with the search, though.
I have heard of seahorse tanks with just tap water that has had the chlorine removed, salt added, and pH elevated to 8.2. If you have coral, etc. in the tank though, you will need to use RO/DI water.
