I've zero experience with barnacles, but some experience with filter feeding critters, and I grow cultured algae, so if you find a species that will survive on algaes, this is a bit of info on how to culture it. I managed to keep fresh water clams alive for over a year using the algae.. though I now think the clams are not a good species for aquaria, I believe that all their needs are not easily met in tanks. Many clams, I know, live only weeks or months, so I did better than some, perhaps. Barnacles might be much the same.. research would be key to meeting their needs.
But on with algae culture. There are a few species you can get, all but one are salt water only. Nannochloropsus is the only one that grows in both salt and fresh. Florida Aqua Farms produces disks of pure cells that live about six months.. giving you time to use them. You soak them in water overnight, gently scrape off the cells from the agar nutrient layer they're on with a sterile swab and add them to a prepared vessel of suitable water.
If you have a salt water species to feed, then you can grow two or three different algae species, which will have varied nutrient profiles to offer the animals.
Sterility in culturing is the ideal, but a bit hard to maintain perfectly at home. I use gallon bottles that come full of distilled water, which are quite clean to start with. I use distilled water, which I think is close enough to being sterile that I don't worry much. I have a small home distiller and use the water fresh from it as soon as it cools enough, but purchased DI
would be ok too. If it has to be salt, you mix up the same salt concentration you have in the tank, or what the instructions for the algae species calls for, ahead of time, since it takes a day or more for the salt to be well dispersed and dissolved and the water aerated. If you keep marine tanks you'll know far more than I do about that, though I am looking to start a brackish tank to raise some larval shrimp that need brackish conditions, at some point.
I rinse the bottles with hot water, already boiled in the kettle, but boiling water in most plastic beverage bottles deforms or shrinks them. To reuse a bottle I soak it in bleach water overnight and rinse well. You can boil tap water or microwave it to sterilize it too, instructions come with the discs of culture.
Cleanliness, covering containers and some care will prevent the cultures becoming too contaminated to feed. I've not had it happen in over a year of growing the stuff. I grow Japanese Chlorella, [Nannochloropsus], in fresh water, to feed larval shrimp, adult filter feeding shrimps and feeders that filter feed, such as ostracods and daphnia, and the clams, while I had them.
You need very bright light for about 18 hours daily. They sell a specific fertilizer, Guillard, and only about 1.6 mls is needed to feed a gallon culture bottle 'til it's ready to feed. Takes 8 - 10 days to be ready to feed or split. I've seen Youtube vids on various methods, some using other fertilizers but I use the Guillard, as it's specific for the purpose of growing algae to use it to feed water living animals. Other ferts may be safe enough, I just have not cared to experiment. The discs are not cheap, neither is the Guillard, but you can reculture almost indefinitely, and one bottle of fert lasts quite a long time.
You must have an air source at the bottom of the bottle. Either a weighted stone on air hose, or run a piece of rigid tubing, cut to the right length, through a hole in the bottle cap. You can put a stone on the rigid tube using a short bit of air hose or just let it run out the tubing end, so long as the water circulates constantly. A hole in the bottle cap is the cleanest way, helps prevent contamination [ other algae spores, moulds etc., which are airborne everywhere ] getting inside the culture. Make the hole just big enough to admit the tubing, seal with a bit of silicone. You can soak the whole set up in bleach/water periodically to clean it.
You can split and reculture more algae from one disc almost indefinitely, and even if a culture crashes, you can often revive it by adding more food and being a bit patient. May happen if you let it run too long, and it runs out of food. So new food may revive the cells that are still alive and eventually reconstitute the culture.
The amount you need to keep your critters alive will dictate how many bottles of a given size you need to have running. You can culture in almost any size bottle that's available or manageable, but it must be clear to let light in. You can refrigerate a culture that's ready, but I'm really not sure how long they will last that way, not having had to do that myself. At least a couple of weeks, based on the bagged variety sold for marine feeding.
So you need space for the bottles, the air pump and hoses and valves to keep water constantly circulating & sufficient lighting that's strong enough. I place my bottles by the side of a light garden I have, with ten 6500 K
tubes on it. The light escaping the sides is enough to grow algae well. They're on a timer, which is useful, I don't have to worry about remembering to turn them on and off.
If you can find out what the maximum size of food item barnacles can eat is, that will give you some idea of what else you might culture to feed them. For example, Freshwater Ghost shrimp larvae need food items of less than five microns in size, and the same is apparently true of most of the larval shrimp species. There is a man made food, Golden Pearls, that comes in sizes starting at 5-50 microns. Originally developed to grow commercial shrimp on farms, they quickly learned that a diet of man made food alone resulted in high mortality rates, far too high to make farming profitable. Turned out the shrimplets need many of the same Omega fats humans need for health. Used to be the standard grow out food was BBS, but they are becoming too costly, harder to source and were eating into profit margins. So they tried for a man made subsitute, and Golden Pearls were the result. 30% of the shrimplet's diet has to be live food, and they now use Moina, mostly, some BBS perhaps, to meet the needs for EFAs. Depending what size food items your barnacles can consume, you might be able to feed something like microworms, or the smaller walter or banana worms, copepods or other infusorians. Several types are available to culture from labs like Carolina, though how much is available for salties, I am not sure. I know micro and banana worms live for days in fresh water, but I don't know how they do in salt. Might find out later on if I get my project going one day.
But if, for example, you were growing marine copepods, you'd still have to culture algae simply to feed the copepods. Be interesting to see where you end up with this, if you find a species that might be able to be kept, with sufficient dedication on your part.
Also, Two Little Fishies makes two foods, one is spray dried algae, the other finely crushed and dried mixed crustaceans, which are intended to be mixed with water and fed to corals in marine aquariums. I use them mixed in fresh to feed both my fan shrimps and the filter feeding feeder species, like ostracods. So far, so good, but a small jar is quite expensive, though at dosage rates for corals it would last for awhile. You can also buy powdered spirulina, which can also be mixed with water and fed in the same way. Spirulina is very nutritious, but whether it's able to make up the needs of barnacles I would not hazard a guess.