Magnum 350 troubleshooting

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Aquarium Advice Apprentice
Jul 16, 2004
Raleigh NC
I have 2 magnum 350 cannister filters on my 75 gallon tank, one running a micron the other just filter media (sometimes carbon if necessary none currently). I neglected to do a filter change on the filter just running the fiber media and it was in dire straights. I imploded my media canister because it was so clogged up. when i cleaned it up and put it back on line though the filter runs backwards, Yes i have plugged it up properly (i have the quickreleases so there is no way to mess it up) I've taken it apart and checked everything over it all looks good. but everytime i restart it it runs backwards. has anyone heard of this happening before and is there a good way to fix it?

You sure it is hooked up right? I have the quick releases and it is possible to hook the hoses backwards (I did it!) - the sticking out bit on the quick release can come out either end so the in & out are actually interchangable.

Make sure the water is coming out the "out" port when you turn it on. If it is coming out the "IN" port, something is screwy. Is the cover on right? ie. you lined up the "out" port to the "out" tube inside the canister? & the gasket is tight?
I have the quick releases on my magnums and I believe it is hooked up properly (it was running properly in the proper direction when i took it down for cleaning). I did not realize that the quick releases where interchangeable (I bought my magnums used and assembled when i bought my tank) So I will check that out. The top is lined up but the water is coming IN through the tube in the cannister to the impeller (i'm almost positive but i'll verify tonight when i get home).

Thanks for the help, I'll check my quick releases and see if that is the problem.
It was the quick releases, I just didn't realize they where interchangeable. Its running like a champ now. Thanks for the insight, I would have driven myself crazy trying to figure out how to fix it without your help.
OK, now I'm really confused, I did a search here to see if I could figure out the problem on the Magnum 350 that was included in the used setup I bought and I see this. Bingo, I must have the hoses backwards. I looked at the manual and thought I had it right but I went to check.

I have the quick releases (at least that's what I think they are, there are two valves on each side and they just twist on and have little clips that lock in place). Well, mine are not interchangable so that's not it (I guess the bottom pieces could be swapped but I never took them off and it looks like they are right from the picture in the manual. (the out port is on the back right and the intake is on the back left?)

After looking at the parts list I realized that I'm missing the gasket that goes on top of the media canister and I'm assuming that is my problem? It appeared to be working at first. I turned it on for 10 seconds, shut it off and it filled with water. When I turned it back on the water level dropped to about 1/2 full and it was only pumping a tiny stream in and out of the tank and looked like it was trying to run backwards like described above only with almost nothing going either way).

Does anyone know if that missing gasket would cause the filter to react like that? I don't want to pay another $10 (with shipping) and wait another week to get the part only to see that it doesn't fix my problem if I can avoid that.
it sounds like you didnt prime it well(filling it up with aquarium water before starting it up) or the missing gasket is causing the filter to louse prime (getting air in it so that it cant fill with water and pump) I would think both could be the case.. you most likely need the gasket and next time fill the filter and hoses with aquarium water before starting it up.. HTH
Thanks, I guess I'll pick up a new set of gaskets and see if that's the problem. I did fill the cannister with water up to the fill line before I started though. I could tell there was air getting in so I'm hoping that's all it is. $10 for new gaskets is a lot better than $100+ for a new cannister.
I did some fiddling this weekend and the hoses were on backwards. The person I bought it from must have switched them at some point. I got it up and running with the micron filter to clean the water up a bit, and then I'll be switching over to the media container with some ceramic rings in it.
Don't get me started on magnums.... I could fill several posts with all my problems and floods. Finally after me and my guinea pigs recieved 2 showers in one day and I scraped/bruised my whole arm the magnum is gone forever. It kept either slow leaking or suddenly breaking apart. This was after the 3hrs of fitting those stupid little rings on the extra thick tubes to keep it from leaking. The last one ended in the tube above the quick connect coming apart and me having to rip the intake off the top of my 90g because I couldn't stop the flow below the tank. Thus my injured arm. My boyfriend came in to find me soaked and all the towels in the house either on the floor or wrapped around the guinea pigs. He just helped me put all the magnum parts in a trashbag without saying a word. That was flood no. 8 after attempting to set the magnum up on 3 different tanks. I should have given up on that thing long ago.
I have owned a Magnum 350 for three years, now it seems to work fine and about 1 hour later the water level falls to half, I have refitted everything several times with the same results. I guess you get what you pay for.
Hey Mike,

You are replying to a 4 year old thread ..... better to start one of your own as people are not likely to notice things in an old thread so you won't get much reponse.

The air trapping is a problem with my Magnum as well. It usually means some gasket is worn out/loose. Try greasing all the gaskets with Petroleum Jelly. If that doesn't work, you can try to find the leaky ones & replace those. <This is usually a problem with the intake side .... maybe the quick release or the intake connection to the canister. One other possibility is that you are sucking air from the tank .... say the intake is next to a bubbler.>
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