If you're not living there and not on the lease they can pound sand.

Most people aren't taught that or ignore it.A bedazzled hammer with jewels and gold leaf will pound a nail just as good as a $4 hammer from home depot.  

We have hardwoods in our personal residence with a 60 pound German Shepherd and I would not want my rental floors to look like my personal house does.

Not only to keep them from being aggressive, but to keep dogs away from the property if a female is in heat, or have one that's not neutered run off and get somebody's poodle preggers.I'd also require renter's insurance with a rider for the dog, if there is such a thing.Ironically, the only breed that has ever attacked dogs of mine, all different types of terriers of around 20 pounds, have been german shepherds.

Tell the tenant to pound sand.

This chart says a 75 gallon aquarium weighs 850 pounds when it's full:http://freshaquarium.about.com/od/aquariumsandstan...So, I'd argue that the weight alone, could result in extensive and expensive damage to your floors/flooring. 

Something you need to verify.I do have an emotional support animal for my disability, but I also rent where dogs are allowed anyway, and I have a dog under 25 pounds, so any place that accepts pets, will accept my dog.  

I don't allow any dog over 25 pounds, so that limits how not only how dangerous they can be, but also limits potential damage.

We allow pets in our condos, in accordance with association regulations - one or two full grown pets under 25 pounds.

There's no visible evidence, aside from the 42 pound bag of cat litter.I did some research and found I can photograph urine stains if I illuminate them with a UV light.