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I have a 55 gallon tank that has a fluval 406 and another 55 gallon tank as a refugium.

I do a 20% water change weekly. Once a month, I open the 406 and rinse the sponges and the biomedia. Setup is as recommended in the the filter thread. The last two weeks have been just water changes, not filter cleaning. I turn the 406 off during these changes, because I drop the water level below the return and it splashes too much if I don't.

For the last two weeks, when I've plugged the fluval back in, there has been no flow--not even any noise. Both times, I've pulled it out, drained it, pulled the top and checked the impeller. Each time, there has been no gunk. It seems to spin easily. I put it all back together, let the water fill the filter, plug it in, and all is fine.

Anyone have any insights. The filter was running on a 20 gallon long when I first got it in August or September. By October, it was on the 55 gallon. If I heard it struggling to filter or I was seeing lots of gunk, it would make more sense. As it is, about the only thing different between not working and working is me pulling the impeller and looking at it and air being evacuated before turning it on.

Do you hear the motor running the first time?

It didn't sound like it this most recent time.

I did try shaking it a bit, thinking perhaps there was air trapped in it. No noise.

Might not have been fully primed. Are you re-priming it before you plug it in or just using the stop valve?

Possible, I suppose. I'm just using the stop valve. There is still water in the unit where I reconnect. When I dumped the water put after it didn't start, it was clearly full.

I'm not doing anything different than I have for months without problems, but the fact that emptying it and reconnecting definitely pushes water through may mean I have some priming related issue that has come up.

As long as it starts eventually, I'm fine. I was just hoping someone had experienced something like this and had some insight.

With the water dropping and the valve off, what ever water is in there might drain down and the filter intake will loose it's prime. When I'm in a pinch, I turn off the filter and use plastic food wrap or sandwich baggie around the intake and throw a rubber band over it. It's and XP3 so there is no stop valve. Then change/refill to previous water levels, remove plastic, then turn it back on.