[Delphinus]

That's interesting .. I wonder if you'll find you need to step up the pump over time though.

My prediction - the first one will deplete faster as it has the nitrates and phosphates coming in. The second one will have water with zero nitrate coming in and at the very least "reduced" phosphate coming in (if not outright zero, but don't know, since I never bothered testing before/after with PO4 anyhow). But it will also have the shed mulm from the first. So it's interesting to see if it will be able to shed these or if you'll need to step up the pump.

Personally although I understand the urge to put "more! MORE!" on because they work so well .. I don't know if I think there's a benefit other than maybe you will not need to top up as much. If the reactor emits water with zero nitrate to begin with (as I found was happening even with less the recommended dosage for my water volume), then the nitrate reducing capacity is not increased with more pellets. Instead the limiting factor is then the water turnover through the reactor compared to the overall tank water. Ie., if you want "more nitrate reduction" then you actually need more water throughput with the effluent still emitting zero. In other words you're probably better off with the two reactors independent of one another, since they will be double the flowrate of one .. but with the two in series you have the same flowrate as one.

My prediction anyhow. Maybe I'm wrong..