the FMJ is discontuniued, since there was no provable incidents, when the magntetic field ever disturbed a HDD.
using the FMJ in comination with a pump that has a fixed blade wheel (e.g. all Aquastream pumps) might even prevent the pump fron starting alltogether. if you use a eheim 1046 or 1048 with a loose blade wheel you can use a FMJ.
- and btw the magnetic field has nothing to do with the grounding of your case