I honestly dont think that the MOLT would cause the nitrates to climb so high... I have had several shrimp as well as crabs molt and my no3
is still at 2ppm.. Have you had an algae problem prior that recently disappeared??? Algae usually thrives off nitrates and lowers them since it is constantly using them.. Therefor making the levels lower..
Water changes is the easiest way to get the levels to drop...
I would say change 20% of your water should cut those back to a better more acceptable level...
Also Some test kits are known to be off. These are usually the cheaper tests.. I personally have had Great Experiences with Either Salifert or Seachem..
Another Thing that might helpful would be pulling the media out of the Magnum and using it as an empty pump as you could be getting nitrate buildup in the filter pad as well... I know that I personally have experienced an issue like with the filter pads and Cartridges.. At least pull the media and clean it real well.. I would definetly wait for someone else to post their opinion about this as you only have 40lbs of LR
which as you probably know is a Great start but most recommend 1.5-2x the water volume to use it as filtration alone.. I must say that I am by far no expert but as well must say that I have been reading and researching for quite some time...