It sounds like your new Mustang has a very healthy appetite, which is a good thing, and I don’t think you need to be concerned about overfeeding your pony. A seahorse’s appetite is a pretty good gauge of how much they need to eat. When they are really hungry, their actions make it known, and you can offer her an extra snack between your normal feedings, if you wish.
As you know, the feeding regimen that generally works best for most captive-bred seahorses is to provide each of them with 2-7 frozen Mysis relicta twice a day, enriched with Vibrance, and then to fast your seahorses entirely once a week. In other words, your adult seahorses should each be eating a total of around 4-14 frozen Mysis each day, depending on the size of the seahorse and the size of the Mysis. But those are just rough guidelines and there is a lot of variation in how much Mysis healthy seahorses eat each day.
A large seahorse naturally eats more than a smaller pony. And jumbo-sized Mysis will fill up a hungry seahorse faster than smaller shrimp. So a seahorse that’s scarfing up king-sized Piscine Energetics Mysis relicta does indeed need to eat fewer shrimp than a pony that’s dining on the tiny Hikari Mysis. (You’ll see what I mean when you get some PE frozen Mysis, Julia — it will probably be quite considerably larger than the Mysis you are using now. The feeding guidelines are based on the jumbo PE Mysis, so you can increase the number of Mysis you feed daily accordingly if you are using another brand of Mysis.)
In short, Julia, it sounds like you have a young Mustang who is still growing and has a hearty appetite, and that you are feeding him the smaller Hikari frozen Mysis (or perhaps the Mini Mysis from H2O Life). In that case, you’ll need to feed him more of the smaller shrimp while your new Mustang is rapidly growing than indicated in the recommendations above. Under the circumstances, I would not hesitate to give your Mustang a third feeding of 4-5 small Mysis each day, if your schedule allows it. If your schedule only permits two feedings a day, then go ahead and give the young Mustang 6-7 of the smaller frozen Mysis at each feeding
Aside from size, some of the other factors that determine how much a seahorse eats are water temperature, the age of the seahorse, and whether or not it is actively breeding at the moment. The warmer the water temperature (within the seahorse’s comfort zone), the higher its metabolism, and the more calories it needs to eat as a result. Young seahorses that are still growing rapidly typically eat more than mature seahorses that have reached their full growth. As you might expect, breeding pairs that are producing brood after brood every few weeks need to eat a lot because so much of their bodily resources go towards producing clutches of eggs or nourishing a pouch full of developing young.
So don’t get hung up trying to count every morsel every seahorse in your tank scarfs down, Julia. Just make sure all your seahorses have full bellies at the end of the day, as indicated by their well-rounded abdomens. After a good feeding, the seahorse’s belly rings should be flush or even slightly convex in cross section when viewed from head on. (We never want to see sunken, severely pinched-in abdomens on our seahorses! Concave belly rings are a sure sign of an underfed seahorse, with the sole exception of a female that has just transferred her eggs.)
I will send you an e-mail and attach a document to it that explains all about feeding Mustangs with frozen Mysis in great detail, including a discussion of the preferred feeding methods, such as target feeding and the use of feeding stations, as well as a discussion of the pros and cons of different brands of frozen Mysis. Just download the attachment, save it on your computer, and you can read through the feeding information at your convenience.
Best wishes with all your fishes, Julia!