Thanks for your question! Freezing your used marinade is a great strategy for saving some resources, but before you use the marinade for a second meal you need to bring it up to a full boil. 

Because the original marinade was in contact with raw fish, there was ample chance for bacteria to grow during the original marinating time. Freezing the marinade stopped those bacteria from growing, but freezing does not kill most pathogens. If you were to reuse the marinade without boiling, you'd be starting off with a large "bacterial load" that will only grow even more during the second marinating time. However, if you bring the marinade up to a full boil first, you will take that bacterial load down to near zero. 

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