oh wait, I don't have one exactly like this one. Mine has only places for the bottles.

If you use PMK or Pyrocat-HD or ABC you just need to have tempered water to mix the chems right before you use them. PMK is 10ml A with 20ml B and 1000ml H2O. I just mix A and B at whatever temp with the correct temp of 1000ml H2O. The PMK developer cannot be stored already mixed as it has a short life after being mixed. I think this also applies to the other pyro developers but I am unclear on this and there are better people to ask than me.