Sorry for a slow reply. This section of the forum (Cuttlefish Care) has some great information available in the stickies at the top for how to raise and care for cuttlefish (assuming you’re raising Sepia bandensis cuttlefish, there’s really not a much better resource out there in my experience). My recommendation is to read through those articles (including this one: Sepia bandensis: husbandry and breeding ) and learn from them. Those articles should cover pretty well everything you need to know, but, right now, with feeding being an immediate concern, I’ll note that the above article recommends live mysis shrimp as a first food. Barring that (if you can’t find any mysis), it recommends trying amphipods. They’re a bit harder for the cuttlefish to hunt, but they should be pretty good nutritionally.
A relevant section from the above article:
Net breeders are also great because they keep hatchling cuttlefish in close proximity to their food. For at least the first 2 of weeks after hatching, Sepia bandensis will need some sort of live food, and keeping the food closer to the hatchlings makes it more likely they will be able to find it to eat it. The more they eat, the faster they will grow, and the sooner you can release them in to their permanent home.
By far, the most successful food for hatchling Sepia bandensis is live mysis shrimp. Mysis are highly nutritious and relatively easy for the hatchlings to catch. The drawback to this food is the expense and the effort. Collecting wild mysis and captive culturing mysis are both extremely labor intensive, so they can cost more than one hundred dollars for 200. I prefer cultured mysis to wild mysis, because in my experience they have better survival rates, but plenty of other cephalopod keepers have had great success with wild mysis.
It is important to note that live brine shrimp, though readily available and inexpensive, are widely considered terrible food for cephalopods. Cephalopods raised on live brine, even enriched live brine, have low survival rates and short lives.
Keeping any live food alive can be challenging, and the challenge is compounded with mysis because they can be cannibalistic. To reduce this potential issue, avoid overcrowding, and be sure to feed rotifers or other suitable food regularly. Net breeders can be utilized, or another small tank can be set up to keep the mysis until they are ready to be fed to the cuttlefish. Its also important to get a feel for how many mysis you need per week, and be able to order them before you run out so your cuttlefish don't go starve or eat each other!
If you are lucky enough to live near the ocean, you may be able to collect your own hatchling cuttle food in the form of small amphipods. Make sure to collect from waters that are as unpolluted as possible, and make sure to check with local regulations regarding collection before beginning. Amphipods can be much more robust than mysis and they can escape from hatchling cuttlefish more easily. I recommend that you start with mysis for the first week or so, allowing your baby cuttlefish to learn hunting skills with the easier prey.
Hatchlings should be fed several times a day, and only as much as they catch in a few minutes. I recommend avoiding 'flood feeding', feeding a lot of live food at once, because not only can hatchling Sepia bandensis stop seeing them as prey items, but flood feeding can make the hatchlings harder to wean onto dead food.
Since live food can be expensive, its great to wean your cuttlefish onto thawed frozen food as soon as possible - frozen mysis are a good choice for size and nutrition. Since cuttlefish rely on their eyesight to hunt, often the dead prey may need to be moving to get the cuttlefish to strike. Start by introducing thawed mysis with your live food. The hatchlings, conditioned to striking when live food is dropped into their breeder net, will usually snap up the dead mysis as well. Sometimes you will have to make the dead prey look alive by gently blowing it around, just barely moving it, with a small pipette or turkey baster. Weaning onto dead prey may not work until the hatchlings have moved off small prey and onto larger prey and determining when your hatchlings are able to move off smaller food is a judgment call.
When your cuttlefish are a month old, and have had time to hone their hunting skill on weaker, smaller food, you can try feeding them larger food…even up to foods the same size as the cuttlefish. "Shore shrimp" or "marine janitors" can be ordered on line in various different sizes, and they make a great food for cuttlefish. Just like mysis, they need to be kept alive until fed to the cuttlefish, so be prepared. Once the cuttlefish are taking larger prey, the weaning process as described above works quickly and well, just instead of using dead myisis, you need to use dead, freshly killed or thawed frozen shrimp.
Another weaning method that cephalopod enthusiasts have been experimenting with is some kind of shrimp hanger or feeding station. Glue or tie a small rock to a piece of fishing line as a sinker. Tie the other end, or secure the other end, above the tank so the sinker will be a couple of inches below the bottom of the tank. In the middle of the line, tie or glue a plastic toothpick, and skewer a dead shrimp onto the toothpick. When you place this device into the tank, the current should make the shrimp on the toothpick move around, which will help attract the cuttlefish to feed. If you have multiple cuttlefish, add more toothpicks to the line for more shrimp.
Weaned or not, as the cuttlefish get bigger you will need to get them larger food items. Again, if you live near the ocean, you can collect local crabs or shrimp as needed. You can also check with local bait shops, which may have live shrimp ready to sell. If you live away from the ocean, you can order live fiddler crabs or appropriately sized shrimp from online vendors. If you have weaned your cuttles onto thawed frozen food, any live food, bought or collected, can be obtained in bulk and frozen to use when needed. Frozen bait shrimp or prawns can also be bought or ordered, and even raw, unshelled and unflavored shrimp from the grocery store can be used.
It is important to note that freshwater feeder fish are not a suitable food source for cuttlefish. Not only do they lack fatty acids of saltwater animals, but they are often treated with copper, and copper is deadly to cephalopods. There is no real consensus among cephalopod enthusiasts regarding the suitability of using freshwater crustaceans as food for saltwater animals like ghost shrimp, so I would suggest limiting their use as cuttlefish food.”
Again, do a bit more research with the sticky articles at the top of the Cuttlefish Care forum and learn the basics of what you need to know from there.
Like I said above, if you can't get mysis shrimp, try and get some amphipods.
From what I've heard other foods (such as copepods, brine shrimp, rotifers, etc.) don't do as well as first feeds, and the baby cuttlefish will die if you try raising them on just those.
One site suggested that you might (no guarantees) be able to raise them on fish fry such as baby guppies (they do have to be babies, the adults are far too large), but I can't find any other information that supports this. I do know that guppies and mollies are the go-to feeder fish for people who keep saltwater predator fish (such as lionfish, anglers/frogfish, etc.), so it seems plausible to me from a nutrition standpoint, but that doesn't necessarily mean it will actually work. Similarly, ghost/grass/shore shrimp (Palaemonetes spp.) fry would seem plausible under the same reasoning. All of that said, however, if you can't find any mysis shrimp or amphipods, trying guppy and/or molly and/or ghost shrimp fry might be better than just accepting that you're going to lose all of the cuttlefish, so I'd suggest giving it a shot. (Hopefully someone with more experience/knowledge will chime in if this idea is doomed to fail and be able to offer other suggestions, but it's the best I've got.)
If you end up trying the guppy/molly/ghost shrimp fry, let us know how it turns out.