I would shut down the 55gal all together and use that fx5 as a house for nothing but media in that 125 gal tank. I would also get rid of the sponge filters and just run air stones/disks. Also you want to make sure that there isn't any "dead spots" in your tank.
Sorry for you lose in fish brother, we've all been there. It's all part of the hobby.
Sorry for you lose in fish brother, we've all been there. It's all part of the hobby.