It has been said this way--the RO is simply one of the pre-filters for the DI section. So, though, percentage wise, it doesn't reduce the TDS figure as much as the RO unit, it is the fundamental section of this type of water purification system.
Sounds like you're doing fine--filtering out the bad stuff, heaters heating the water to 80.6. I presume you know to wait for your nitrite reading to go to zero after your ammonia reading has gone to zero.
I don't think you have given enough information for anyone to be able to help.
What size is your tank, what rating are your heaters, how cold does it get at night where the tank/sump is? What are you using for a skimmer? What is the salinity or specific gravity of your water? Have you added...
I've never experienced an Eheim but I have a couple of Aqua Medic OR3500s and OR2500s that I am exceptionally pleased with, including their lack of noise. (I had Iwaki's previously--oy vey! 8O ) I don't believe an OR3500 would be enough pump for your 120 with dual overflows (two of them...
Talkingreef had a really neat extended podcast about three years ago on keeping seahorses. I listened to it recently and would recommend you listen to it.
http://www.talkingreef.com/forums/podca ... -89-a.html