The check valves stop the solution from flowing backwards out of the tube. When this happens, the doser turns on for the set time, but has to pull the fluid up the tube again before it starts coming out the end. This means it wont dispense the correct amount of fluid, or none at all. Usually you only need them for some of the cheap 3 roller pump heads. The better 4 roller heads generally dont have that issue.
Have your Alk numbers been fluctuating like that alot? The loss of corals might be more from fluctuating alk rather than low alk.
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