I noticed an increase in sickness when I used the seahorse too. But maybe that's because I didn't know he was sick. Of course I was treating the other fish with him, so he should have been treated too.
I honestly have not found a positive use for him. I think it would be cool if he could mate a fish causing a random baby. Kind of like the Certain Magic FIsh, that I won't name and spoil things, but it would only affect that one fish and not the whole tank.
I've read some people say that by buying him you can store more fish in each tank. That's just not true. I can store 21 fish in each tank from the beginning.
So is there really a reason to buy him? Maybe some one from LDW could give us a hint? Please?

Marcey

we have tried begging

we have tried bribing

we have tried badgering

no dice

I got more sick fish, too. But then, I didn't know she could get sick. Still, it's nice to know that spending the money on her will do something. Plus, knowing she does something means I still have new things to discover.

I've noticed that the older my fish are, the more prone they are to being sick, especially with the magic fish since I will always keep a breeding pair. When they start getting older, I have been replacing them with younger ones and selling the old ones and that seems to help.

What I've discovered with the seahorse is that it helped keep my rare babies alive longer before I upgraded my tank. It seemed to work the the adults who wouldn't quite thrive in my regular tank, as well.

Without the seahorse in the tank, I'd get a baby that would die as soon as it hit 5. With the seahorse, it'd live through part of 5 and maybe hit 6 before dying. I didn't really find it that useful though, as any adult that looked like it was fragile I sold off to fund a tank upgrade.