Digital BW, The Print

I set up my 7600 for b&w printing. Currently I'm using 6 channels and 
I'm planning to keep it this way for a while. I've just noticed though 
that the 7th channel, which is not actively used, is beginning to 
develop a clog. I want to remove the ink from that cart (funnel fill 
type) and replace it with some kind of fluid to keep the nozzles clean 
and going. 
What type of fluid do people normally use in this situation?

Thx
Dario