Why might it be that my vitamin d level be low when i take fish oil?

Poor source of D: Most fish oil has little or no vitamin d. Cod liver oil can be a good source of vit d but most brands of cod liver oil have had the vit. D mostly or entirely removed. Most adults need 5-8000 iu of vit d a day for optimal levels. Read the label of your fish oid to see how much d (if any) it contains.If it does not list d on the label it probably does not have any! take d3, not d2 which is dangerous.