I was just digging up some links for another thread, and came across
Randy Holmes-Farley's comments on what passes through RO membranes:
"However, at the small end of the spectrum a number of compounds can pass through a reverse osmosis membrane to some extent and are, therefore, of concern to reef aquarists. These include carbon dioxide (CO
2), ammonia (NH
3), hydrogen sulfide (H
2S, especially a concern with well water) and
silicic acid (Si(OH)
4, which is the uncharged and predominate form of silicate at pH values below 9.5). All of these should be trapped by a functioning DI resin (discussed below), but can still be a concern."
Obviously, there isn't a 100% fail rate with these compounds either though.