[Delphinus]

Are you backflushing the membrane periodically?

It depends on your source water how long your prefilters will last. I find I get a few months out of mine, my source water TDS is around 180-200 I think. 6 is not an uncommon reading for the RO output, but I find in my case usually what happens is it's more of an initial surge right after turning it on, it settles down to zero after 30-60 seconds.

DI output is always zero though unless it's spent. Ie., if you're getting 6 after your DI then it's time to change the DI. You should be getting way more than a month out of your DI. I don't have an Aquasafe unit but I get at least 6 months out of my DI cartridges (to give you an idea of what is reasonable to expect).

One note, is your unit directly plumbed to a float valve on your sump? Because if it is, this is very hard on a RO membrane. The constant on/off cycles is very hard on the membrane. I found this out when I had to replace mine 3 times in a year. It's better to have a reservoir that feeds the float valve on-demand as per evaporation, and fill the reservoir in a single go when it gets empty.