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I've got a Magnum 350 on my 90 gallon turtle tank. The water is about 3 inches from the top of the tank. My problem is whenever I fill my Magnum 350 up with water, all it does it pump the water back into the tank until there is no more water in the filter, it does not suck any water into the filter. Any ideas on what to do?

Start by making sure it's put together right and the intake tube is clean.

I checked the tube, and my friend that has one came over, and he said it was put together correctly, and didn't know why it wasn't taking in water.

is there a check valve that could be placed backwards or stuck? it's odd that it pumps out but doesn't suck any up. You've got the filter tube in the water, right?

Yes, the intake tube is in the water. I'll look at it some more tonight.

Tonight I hooked a small pump onto the intake line of the filter to get water flowing into the pump, then I turned on the filter. Then I turned off the pump, and pulled it away from the intake line, and the water continued to flow. But, the filter was taking in more water than it was pumping out, so water started spraying out of the canister. I turned the filter off, but the water kept coming, so I pulled the intake out of the water to stop the water. It made a big mess. Any ideas on what to do?

The top section of the filter is not seating or sealing properly.

Carefully check the condition of the big O ring that fits around the rim of the top black part. The O ring must be round in shape, not flattened, and must not have cracks in it, or cat hairs etc stuck on it. Any of the above things will prevent the O ring from forming a perfect seal. An imperfect seal means a leaking seal - water and/or air will bypass the seal and can casue both the problems you have described (failure to draw and water spraying out).

Replace the O ring if its missing or defective. Gently wipe it clean if theres any hair on it. If there are any tiny cracks in the ring, a temporary fix can be made by applying a very thin coat of silicone lubricant grease.

Now, when you are placing the black top part onto the clear cylinder, make sure to align the notch before you carefully push the black top part onto the clear cylinder and do up the silver clips. It is never required to use any force to do up those clips. Forcing will crack the attachment points for the clips on the cylinder. If you have placed the black top part on correctly and pushed it fully down onto the clear cylinder, the clips will do up easily, no force required.

Have the cylinder filled to just the water line. Turn the pump on and let it pump about 30 seconds. Turn it off and water should flow down into the cylinder displacing out all of the air above the fill line. NO water should leak out anywhere, except maybe for a trickle down the side of the tank if big bubbles from the burp splash water. Now turn it on again and let it work.

Bill:
Some things to check...

If the filter is not taking in water then you have a blockage somewhere in the intake subsystem. The Magnum uses a double-valve disconnect system. Make sure both valve levers are pointing vertically (up-down) and not horizontally (sideways).

If you can't blow through the disconnected hose, then you have a blockage in the hose. Locate and clear the blockage. If this is a newly-purchased unit, the have the hose(s)replaced under warrenty.
Check all hose attachments. They're a little goofy with those compression rings. Recheck the assembly instructions to make sure you indeed do it right.

Remove the filter sleeve and any media in the inner basket and recheck water flow. Perhaps the inner basket is overfilled and water cannot pass through it?
Check the o-ring, gasket, and innards.
The rubber gasket goes between the black update tube assembly and the inner basket.
The top of the black uptake tube (this is the black tube assembly inside the cannister) seats in a cylindrical slot in the lid. Make sure it is. I'll check the next time I service my filter, but I'm certain there's a gasket at the top of the black tube which goes into the slot in the lid.

To initial-prime the cannister, fill the cannister with water to the line. With the outtake below the water line in the tank, open both outlet valvles. The cannister should fill with water. Now open the intake valves. It should 'burp' out trapped air in the intake hose. Turn on the filter. The motor/impellor is powerful enough to self-prime at this point.

Overpressurizing the filter with an external pump probably caused the leakage. The filter is plastic and can only stand up to so much pressure. The o-ring can easily slip off when you're reassembling the lid, so make sure it's where it's supposed to be and not floating around inside the filter.

-Wayne