In this case, I would wait on the scooter and introduce the seahorses to your aquarium first.
When discussing compatible tankmates for seahorses, it’s important to remember that one can only speak in generalities, Joe. There are no unbreakable rules, no sure things, no absolute guarantees. For instance, most hobbyists will tell you that small scooter blennies make great tankmates for seahorses and 9 times out of 10 they’re right. But every once in a while, you will hear horror stories from hobbyists about how their scooter blenny coexisted peacefully with their seahorses for several months and then suddenly went “rouge” overnight for no apparent reason and turned on the seahorses, inflicting serious damage before it could be captured and removed.
Does that mean that we should cross scooter blennies off our list of compatible tankmates for seahorses? Nope — it just means that we must be aware that individuals within a species sometimes vary in their behavior and respond differently than you would expect, so there are exceptions to every rule. It’s fair to say that scooter blennies generally make wonderful companions for seahorses, but there’s always a small chance you might get Satan reincarnated in the form of a scooter blenny. There’s no guarantee that adorable scooter you picked out at your LFS because of his amusing antics and puppy-dog personality won’t turn out to be the blenny from hell once you release him in your seahorse setup.
Over the years, I have heard a number of reports from home hobbyists about seahorses that were killed or injured by renegade scooter blennies, Joe, so that’s something you should also be aware of, sir. When the scooters become aggressive, it is by no means restricted to competition for food – rather, the scooter blennies physically assault the seahorses and inflict serious injuries, which are sometimes fatal. The diabolical thing about scooters when they go bad is that they are very devious about it. Many times they are perfect gentleman during the daylight hours, particularly if they know they are being observed, and they will often do their bullying and beating up under the cover of darkness, or when they are confident no one is around. They tend to carry out hit-and-run assaults, inflicting a quick, drive-by beating on their unfortunate victim when they don’t think they’re being watched, and then settling down again for long periods after their brief flurry of hostility.
When that’s going on, often the only sign that will tip off an alert aquarist before it’s too late is a tattered or torn dorsal fin or other injury to one or more of the seahorses for no apparent reason. If you see such signs of damage, you can be sure one of the ponies’ tankmates is up to no good, and a scooter blenny running amok is something that has been reported to me several times by seahorse keepers.
But as I said, the chances are good that your scooter will remain well behaved and never cause any significant problems. However, to reduce the chances of any potential problems in that regard, I would introduce the seahorses to the aquarium well ahead of the blenny, so that the ponies are the established residents in the tank and the scooter is the newcomer.
Nowadays, after learning about the negative experiences some home hobbyists have had with scooters, which often resulted in the loss of one or more seahorses, I no longer take any chances with scooter blennies and if personally exclude them from my seahorse tanks to eliminate the possibility that I might have one of the uncommon bad-tempered blennies…
Best wishes with all your fishes, Joe.
Pete Giwojna, Ocean Rider Tech Support