I've been playing 2 identical games simultaneously. One with the seahorse and one without. I haven't researched any upgrades for either game.
I haven't found any differences between the two, though 1. Delicate fish deterioate at the same rate, regardless of whether the seahorse is in the tank or not.
2. I've made fish sick by overfeeding. Both games take exactly the same amount of food before the fish get sick.
3. Fish do not heal quicker because of the seahorse.
4. Fish do not sell quicker if the seahorse is in the selling tank.
5. Fish do not sell for more money if the seahorse is in the tank.
6. Fish do not grow faster nor have more babies if the seahorse is present.
I'm flumuxed. I don't believe there is an advantage to owning the seahorse. Other than it looks quite pretty in the tank.
So, LDW, if there is an advantage then please put us all out of our misery!