RO/DI Unit Installation problem

I'm trying to install a 5 stage RO/DI unit that I bought from EBAY....the
instructions cover a basic kitchen install but don't touch on fish room
applications. There are 3 lines out that I'm a bit confused as to where
they go.

One goes to the reservoir tank....
One is the RO line out...
One is the DI out...this is the one that has me confused. I have no
experience with a unit that incorporates a DI cartridge. I thought that
there would only be one line out for the water into the tank/sump. All of
the cartridges and fittings are connected so if the RO line out goes to the
sump, where do you run the DI line out?

Thanks in advance....I hate projects that I can't figure out using basic
common sense.