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It was working before last time I emptied the tank. After I put all the water back in it would only suck halfway up the intake tube. So I washed off the motor and now it doesnt suck at all, it just makes a loud grinding sound. How should I fix this? (I just got it last week!)

You are supposed to prime the filter by filling it with water before you turn it on.

I have had the same problem with the aqua clear. Like jozzep said fill it with water. Also get a pen or some thin object and give the propeller a tap while it is turned on. That should get the propeller and the filter working again.

Also make sure that there isn't any exposed place where air could be getting in the tube. I have an extension on my Aquaclear 500 tubing that isn't air tight. If the water drops below a certain level, the filter will behave just as you're describing.

After water changes, even with the filter primed (filled with water) I sometimes have to fiddle with the flow adjuster level to coax it to take up the water.

Yea, it sounda like air getting sucked in from a connecting place. This is why i use aquarium sealer and seal all connecting area. It makes power filter work in really low amounts of water.