Be careful, if the tank water is due to one of your mistakes then you are SOL, most insurance companies use a standard ISO ho-3 contract , they all just call it something different , let's say you had your tank on a stand or area that was not level, well that would fall under negligence and therefore not be covered. You don't need a rider for your equipment your agent is just getting more $ from you that way. You will never get your livestock covered .ever. But your equipment is covered regardless , unless the damage to it is excluded in your policy.
Slow leak from tank that ends up ruining flooring , not covered.
If you do sustain water damage though from your tank it will cover all personal property damaged and any structural damage unless the leak of water was a specifically excluded peril.
Pick my brain for anything else , I'm a senior underwriter w/cpcu so I can pretty much answer any specific ?'s you might have.