Is your apartment building a large steel and concrete building or a wood framed building? If it is wood framed, there is no way it is going to support the weight of a 300g tank. The number reason for tank failure is due to the tank not being level which is either caused by the not leveling the tank or the floor supporting the tank gets warped from the weight.
Even in in a steel and concrete building you can only truly check if you understand how the building was designed. I went through this when I was in a steel and concrete building and ended up with just a 75g. In my current home I have a 535g tank but its strictly because the tank is sitting on concrete slab of the home.
May I ask more about how you determined 75g was max for your building?
I'm in a similar situation to OP. Live in a condo with concrete flooring. I've had a 125g which I want to replace with an acrylic tank. New tank would go in same spot. Wall behind it separates condo from hallway.
I've researched this on forums and had difficulty figuring anything out. I remember a thread where someone suggested to person to get structural engineer but I was swayed by a response that no one is gonna sign off on situation for just a small consulting fee. You'd pay a lot if they were going to assume liability for their opinion.
I'm trying to figure out what maximum tank I could get for this space. Some speculated that concrete flooring would give you a lot of leeway but your posts makes me nervous.