Most commercial fish foods contain various forms of astaxanthin, some natural such as the microalgae Haematococcus pluvalis, some use synthetic forms such as carophyll pink/red. If you are feeding a quality commercial fish food adding additional astaxanthin to that diet will be a waste of money, and could cause some of ones fish to become an unnatural coloration, such as turning a fish that is naturally yellow, to orange. When using elevated levels of synthetic forms such as carophyll pink/red, there could be additional long term health risks.
Approx 10 years ago I ran experiments using elevated levels of astaxanthin (Haematococcus pluvalis) from one of the largest suppliers in the US (Cyanotech corp) against the NLS brand, and I found no difference among the test subjects. If one is simply feeding a homemade food as a staple then it would be a good addition to the mix, otherwise IME a waste of money.