I would use the fish tank sealant.

You see, the reason they don't recommend using ordinary silicone for making or repairing aquariums is because ordinary silicone caulk will contain mildewcides. The way mildewcides work in silicone caulk is exactly the same way they work in mildew resistant latex paints; the mildewcide has such a high affinity for water, that it migrates through the caulk or paint to the surface, where it kills any mildew spores before they have a chance to grow.

The problem with using regular silicone on an aquarium, is that the mildewcide leaching out of the caulk could kill the fish. The amounts we're talking about are small, so I doubt it would have any effect on a horse, but for your own peace of mind, better to use a caulk meant for fish tanks.

I'm sure you could buy a fish tank silicone in any pet store, especially those that cater to people that keep tropical fish.