Well, sir, providing the aquarium has an abundant supply of copepods and amphipods the seahorses can graze on, fasting them for several days is feasible as long as it’s not a regular occurrence. So not feeding your seahorses for a long weekend (say a four-day weekend) is an option, but not feeding them for an entire week at a time is not advisable. There are better options you can consider if you will be away on a trip or vacation for a week or more, as discussed below:
Unfortunately, there aren’t too many really good options for those times when you’re going to be out of town for an extended period. Automatic feeders just aren’t feasible for the frozen foods or live prey seahorses require.
When you’re going away, underfeeding is vastly preferable to overfeeding. Your seahorses (and aquarium fish, in general) can fast for a long weekend with no problem at all, but that’s not an option when you’re going to be gone for a week or more.
I would recommend ordering some hardy live feeder shrimp, some of which can safely be added to the tank every couple days, knowing they will survive in the aquarium until eaten. Ocean Rider’s red feeder shrimp (Red Iron Horse Feed, Halocaridina rubra) are ideal for this once you have gradually acclimated them to full-strength saltwater. They are what I’d like to call a "feed-and-forget" food. They are tough, rugged little shrimp that you can toss in your tank with no acclimation whatsoever. They are agile and elusive enough that your filters won’t eat them and the seahorses won’t be able to capture them all right away. Some will hide and evade well enough that your seahorses will still be hunting down the stragglers for the next day or two. Best of all, you can toss a big batch of them in your aquarium, secure in the knowledge that they won’t perish and pollute it, but thrive and survive as real, live, "catch-me-if-you-can" prey items that seahorses cannot resist. Nothing stimulates a seahorse’s feeding instinct like the frantic movements and evasive maneuvers of natural, living prey.
So if you are planning on being away for an extended period of time, Laurence, I would suggest ordering a 100-500 count of Red Iron Horse Feed in advance, and setting them up in a small tank of their own with a small algae-covered live rock, and gradually adjusting them to full-strength saltwater before you leave. That way, when your "fish sitter" checks in on your aquarium every so often, he can just add another netful of red feeder shrimp to the tank and that should take care of your seahorses’ feeding requirements until he stops by again.
Aside from adding the live shrimp, your fish sitter’s duties will be extremely simple, mainly just checking to see that everything is operating properly. Power outages, equipment failures, or the untimely death of a specimen can wipe out your tank if they happen while you’re away. Consider recruiting a friend, neighbor or coworker to look in on your tank at least a few times while you’re gone. They needn’t be aquarium savvy at all, since you will be assigning them only the simplest of tasks: (1) check to see if the equipment is on and operating properly; (2) add freshwater to replace water lost via evaporation; and (3) toss in a netful of live red feeder shrimp. And that’s all. In fact, it’s a good idea to forbid them to touch anything or do anything other than those 3 very basic duties. And, of course, you will be familiarizing them with your setup beforehand, leaving them a plastic gallon jug of dechlorinated tapwater or RO/DI water with which to top off the tank, and providing a supply of red feeder and a net so all they have to do is scoop some of the shrimp and dump them in the tank. (Stick with the live food if you recruit a fish sitter, Lisa. That way they won’t have to deal with preparing frozen mysis and there’s no danger they’ll overfeed it. I learned the hard way that inexperienced seahorse keepers ALWAYS have a tendency to grossly overfeed, but that’s not a concern with red feeder shrimp.)
Ocean Rider’s red feeder shrimp are easy to keep on hand. They are extremely hardy and very easy to care for. They can be kept indefinitely in a spare 2-10 gallon tank, or even a clean, plastic bucket, that has be filled with clean saltwater and equipped with an airstone for aeration. Neither a heater nor a fancy filtration system is required. They thrive at room temp and tolerate a very wide range of salinity and all they require is an airstone (or a simple air-operated foam filter at most) to keep the water oxygenated, with perhaps a little coral rubble as substrate and a clump or two of macroalgae (sea lettuce, Ogo, Gracilaria) to shelter in and dine upon.
Don’t worry that your seahorses will get spoiled on the live food while you’re away and need to be retrained to eat frozen Mysis when you return. That just won’t happen with Ocean Riders. They are so accustomed to eating frozen Mysis as their staple diet that it can sometimes be difficult to get them to accept live foods. Those red feeder shrimp from Hawaii (a.k.a. Volcano shrimp) are one treat the can’t resist, however. They will go right back to eating frozen Mysis without skipping a beat when you return.
Another option would be to hire a local aquarium maintenance business to service your seahorse setup a few times while you’re gone. Having professionals service your tank can be expensive, Lisa, but it shouldn’t be too costly as long as it’s just for a few service calls on those rare occasions when you’ll be away for an extended period, and it’s mighty reassuring to know your fish are in good hands. Might be worth it to have that peace of mind.
Best of luck with your seahorses while you’re away from home, Laurence!
Happy Trails & Happy Holidays!