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Can I use the valves atop the filter to slow the flow though right down without overloading the pump?
I should know the answer to this but cant get my head around whether it would.

Just how much is "right down". Obviously, if you turn it to "no flow at all", the pump will probaly overheat. If you turn it down to about half or so, should fine. Aside from overheating, I don't think there is a way you can damage the pump by overloading it. Since it's "watercooled", you need water flowing through the pump head fast enough to keep the temperature down.

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Mats

Not all water pumps are designed to be run at full capacity, others require it. It might be a better choice to "tee" the water line and bleed off the water pressure you do not want. I would worry putting back pressure (by restricting flow) on the pump would cause it to overheat, as the pump generates a consistent exact amount of heat and requires a necessary water flow to reduce that heat. But I am not familiar with FX5s.

I specifically am asking because the FX5, I believe I could be wrong and got it mixed up, a simple microprocessor that does something to the pump outflow when the filter starts to get blocked or something.

Yeah, I just found a reference to the FX5 having a "circuit" to check the pump. So the suggestion of a TEE is a good one. Just connect the outlet to the intake with a valve inline. That way, the pump will run fine, and full flow, but you only get SOME of the flow into the tank.

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Mats

cool, thanks.

I'm sure most fluval filters are fine running at a max of 1/2 the output. You could always email Hagen/Fluval & ask them as they are very helpfull or have been when i've asked them queastions in the past but it can take about 2 days for them to get back to you but they do get back to you