Well, hopefully someone with more experience/better ideas will chime in, but some of the methods that I've heard include the following:
- Put the food in a zip lock bag, add water, seal the bag, and flip the bag over and over on a counter for a couple of minutes. After that, flip the bag over a few more times every few minutes. After ~10 minutes let the bag sit ~2 hours. They say that if you added the right amount of water, then the feed can be squeezed with your finger to the consistency of firm clay. If you've added too much water, then the feed will mushy, and if you haven't added enough, then the feed will largely still be dry. They recommended a roughly 1 to 3 ratio of water to feed, but noted it will likely change based on the feed used.
- Mix the powder with some water and basically spray or drizzle it onto the pellets, then either let it dry naturally or bake it at low temps to help it dry.
- Similar to the method above, mix the powder with some water into a paste, add the pellets to the mix and either let it dry naturally or bake it at low temps to help it dry.
- Mix the powder with gelatin to basically make a gel paste to spread on the pellets (possibly also baking at low temps to dry out again).
- Powder the pellets, mix the pellet powder with the supplement (spirulina, astaxanthin, etc.) powder, and turn the mix into either a new pellet, a gel food, or a frozen food to feed. (This method and the method below would likely be quite difficult to get the sizing right - with the other methods, if the supplemented feed is too large, you can just switch to smaller pellets to supplement.)
- Turn the supplement powder into a pellet or gel food and feed separately.
That's all I've got for now.