From my understanding yes it will help with ammonia, but the efficiency depends on your water hardness. Make sure you use the post filter cartridge that contains cation resin, and the two carbon prefilters. The R/O makes the source into ammonium, then the cation resin removes it. Not sure exactly how efficiently, as 4 sounds very high. I know everything exhausts over time (carbon, membranes, cation resin) so it will need to be replaced. Test the water in and as it comes out then compare. R/O wastes around 10% of water it processes, you can daisy chain the waste line from one unit to another to cut the waste down significantly.
The potential problem with using 100% R/O is that you've removed so much of the mineral content from the water you could cause a crash from having the nearly pure H20 being added from the R/O system. I would consider hardening it a little, maybe add some aragonite sand and/or oyster shells to the filter media. I wouldn't go overboard, but you don't want to get into gh/kh/ph imbalance. Some say that the minerals added from the food is enough, but I'm uncertain here as I've heard of bio crashes blamed on straight R/O and I don't have direct experience using it at 100%.
Are you sure it's ammonia and not Chloramines? Tests can sometimes confuse them. Do they use Chloramines in Indonesia? If so using safe or prime first will help greatly.
From the internets, cut and paste is easiest here:
Reverse osmosis units with at least two carbon prefilters should remove chloramine nicely, but ammonia must still be dealt with.
According to Francis S. DeSilva ("Supplying DI Water for the Aquarium Industry") from
Water Technologymagazine's website:
Ammonia can be present in water in two forms, either ammonium hydroxide (NH3) or as the ammonium ion (NH4).
When the pH of the water is less than 7 the ammonia is present as the ammonium ion. As pH increases above 7, more of the ammonia is present as ammonium hydroxide.
The ammonium ion is readily removed by cation resin. Good removal capacity can be expected in waters low in hardness. Waters that are high in hardness will have decreased capacity due to the simultaneous affinity and removal of calcium, magnesium and the ammonium ion.
What this means is that a postfilter cartridge containing standard water softener resin should remove ammonia nicely.
Reverse osmosis removes the hardness and decreases the pH, assuring that the ammonia exists in the ammonium ion state that is readily removed by standard cation (water softening) resin.
Good info here also:
http://www.who.int/water_sanitation_health/dwq/ammonia.pdf
