Well....here are my results.

I used two tanks. One with a seahorse, one without. No upgrades were researched. The tanks contained similar species of fish. Both tanks were full. (20 fish with seahorse, 21 without the seahorse).

I starved my fish in both my seahorse and non-seahorse tanks.

Firstly, I've noticed that when the 'Fed' bar is empty the fish don't die. It makes no difference if there is a seahorse in the tank or not, they still don't die.
When fish are starving then their health deterioates. This happened at an equal rate between the two tanks. Evenutally, when the health bar became empty the fish died.

The number of fish that caught an Ick or Fungal disease were evenly distributed between the two tanks.

Finally, the fish died at an equal rate.

I conlude that the fish in the tank that contained the seahorse were neither healthier nor lived longer than those in the tank without the seahorse.

happybluebird - I know you experienced a different situation in your game. I think that it was a coincidence that the fish with the seahorse survived and the fish without the seahorse didn't. The fish can survive for several human days without food (I played on 2x normal speed).

If none of the fish become ill then you would expect the fish to live for a considerable amount of time. Only if one or more fish catch a disease will they die more quickly. Once one fish has a disease, the others are more prone to catch it too.

If both of your tanks were full then you'd have had 20 fish in the seahorse tank and 21 fish in the tank without the seahorse. The fish without the seahorse were 5% more likely to catch a disease than the ones with the seahorse. This has nothing to do with the seahorse, but merely that there would be 5% more fish in the tank without the seahorse.

Another factor that may have affected your outcome could have been the stage in which you abandoned the game. If the fish in the tank without the seahorse had been left for longer - i.e. you exited the game having last fed the fish in the tank containing the seahorse then you would expect these fish to be in better health than those in the other tank(s).

Having said all of this, I'm still at a loss as to what the seahorse actually does.