A 24 gpd unit is pretty low capacity - my guess is that you are a very patient man.

Some things to look for in a good reef RODI system:

Standard-sized prefilters, membrane, and DI cartridge
Brand name, high-rejection membrane
Specifications provided for each stage
Pressure gauge after the prefilters and before the membrane
Thermometer
DI bypass
Vertical DI stage
Refillable DI cartridge
Aluminum bracket
Quick connect fittings
Flush valve
Clear housings
High-quality instructions
Customer support after your purchase

Let us know if you have questions.

Russ