Carbon Reactor and Calcium reactor

Would be a hassel to change carbon. Would have to tweak your ca reactor everytime. Pluss you would have hardly any flow going through the carbon. Sounds like a bad idea.

 

I'm an advocate of buying a larger, over-sized return pump, then running all of your reactors off of your return line. If you do this, then it's not as big of a budget imposition to have a backup return pump on hand in case of a pump failure. Not only will this typically result in less energy usage (assuming you select an energy efficient pump), but it will also better prepare you for an emergency if your return pump failed.   But do not put them inline (i.e. water goes thru carbon reactor, then GFO reactor, etc), run them all as individuals as they usually require different flow rates.
--AJ