Having used the Silver Magnet for several years (still filling up the first one) I would suggest a separate container to keep things clean. (I'm pretty sure I posted a photo of my setup somewhere in one of these threads.) I use a plastic shoe box with a snap cover to keep the dust out then I pour the fixer through a coffee filter before re-use. I don't know how the fixer is affected by the electrolysis other than if you keep the fixer in long enough that the remaining silver will not register on the test strips, it clears film very quickly. I have seen no issues at all with fixer I've re-used after treating with the Silver Magnet, but after the third time through I take it to the local household chemical drop off just to make me feel better.
If you get a Silver Magnet, get a pack of test strips as well (I tear them in half as you don't need a full length strip).