I was wondering, just now I bought the magical seahorse, but it only stays in one tank. Does it switch from tank to tank over time? Does it enhance the environment for all tanks? I was wondering this because he's only in my first tank.

Excuse me that's the Golden Seahorse.
Also, I think it is stupid that it requires a L3 environment. The poor seahorse has very low health. What a waste of 1300$

Well, for some reason it's health is back at full, and it needs feeding every once in awhile, but I can't figure out what it does. Or how it is supposed to be fully installed but stays in one tank. Can someone help me?

The seahorse, like the tank ornaments, can be in only one tank at a time. And, like the tank ornaments, it can be moved from tank to tank. There has been a lot of speculation about what the seahorse does, but nobody has figured it out for sure. There's a discussion about the mystery of the seahorse in this thread.

Thanks for the link. Assuming the one person who posted said "The seahorse supposedly extends the life of the fish roughly 50 game years" or something like that, is right, then the seahorse has no use to me because my environment is fully researched. Then again the seahorse might be the reason I have never had a fish die, and have had only 1 disease.
They should put "Installs into current tank" instead of "Comes fully installed" because really, the seahorse installs into current tank.