My question would be, weight can you put on it and could it trigger any legal issues for a land lord?

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. 

You need to weight the cost, quality of tenant and long term value it adds.

There is no guarantee that the floor will hold the weight of the bed and if that thing for some reason bursts could cause a fire.Now back to Emmanuella - there are many things you are not saying, once again.

As their stuff was being carried to the curb I tried to explain it to the 6'3" 250# 19yo weight lifter (not the point your thinking) why his and his 18yo special needs brother's stuff was being placed in the street.

The sheer magnitude of information stored there actually added half a gram of weight to my PC.Would much prefer the interaction available here on a more recent post.

(Before you've dug and poured your sixth hole the first poured hole is nearly ready to accept weight) *Build your new girders with glue/screws/ 1/2" plywood sandwiched 2x10's (will NEVER sag, plywood mandatory, emphasis on the word "mandatory").

Great tenants are worth their weight in gold!

For pets, you'll have to know which breeds are unacceptable by your insurance carrier for coverage, set a limit on weight and require they show proof of current vaccination and registration.

I like much of the advice above Amanda, and if you decide to allow pets, you might adopt a "pets on approval" policy with some criteria (number, breed, weights, species, etc).