Reddit mentions of Fritz Aquatics PRO - F/2 Algae Food PTB - 16oz

I don't know about kelp specifically but you can grow microalgae. Macroalgae is only a small part of the photosynthetic life in the ocean, microalgae and plankton play a big role in converting CO2 into oxygen through photosynthesis.

Microalgae like chlorella, spirulina, polysiphonia, etc, convert carbon dioxide into oxygen through photosynthesis. Plankton do the same. There are endoplankton which eat other plankton for nutrition, but there are also phytoplankton that produce their own food through photosynthesis. The specific names slip my mind but there are green and gold ones.

You can purchase algae food here and here. You need equal parts of both, a teaspoon each per 10 gallons of water. Make sure you use filtered (preferably distilled) water, do NOT use tap water. (this kills the algae)

Spirulina and Chlorella can grow in fresh water or alkaline water, polysiphonia can grow in salt water so feel free to add ocean salt to your polysiphonia growing tank. (you can just use a fish tank or a mason jar, a bottle of water if it's clear and can have light penetrate. fish tank is probably best because it's hard to measure a tenth of a teaspoon)

here is a link to ocean salt.

The green and gold phytoplankton in this pack are photosynthetic and can grow if you give them minerals (algae food) and light (sunlight works or you could purchase a grow light) Not sure if the bottles in the link are live cultures but those are the types of plankton you can grow. (names are on the bottle)

You can purchase live algae cultures of spirulina, chlorella, and polysiphonia in the links provided. Just add light, water, and minerals, and they'll begin to replicate, grow, and turn their minerals into amino acids and essential fatty acids. I'm sure they'll clean up the air in your room, and you can also use them for food. (very nutritious)

That's basically how you grow them. I hope that helps, if you want to go johnny appleseed and plant algae all over the world to clean the atmosphere I'm not sure how to help with that. If you live in a sunny area and don't have stupid building codes then you could dig a pond in your backyard, maybe throw some koi fish in there, make a nice waterfall.