A billion is a lot of money, you could do absolutely whatever you wanted.
I would definitely have a few hundred gallon reef tank filled with sustainably aquacultured corals and fish. Maybe break that rule for a couple special fish. Would pay a good company to care for that tank.
Would also build a fish room with a bunch of different setups. Designing, setting up and breaking down the tanks is a fun part of the hobby for me, so I would rather have a bunch of smaller (75-180) tanks than a couple huge ones that never change much. Also this would keep maintenance manageable, as I would rather still care for the smaller tanks myself than pay someone to care for all of them. It gives you a sense of pride when you've done all the maintenance for a fish and you watch them grow up in front of you.
I would definitely have a few hundred gallon reef tank filled with sustainably aquacultured corals and fish. Maybe break that rule for a couple special fish. Would pay a good company to care for that tank.
Would also build a fish room with a bunch of different setups. Designing, setting up and breaking down the tanks is a fun part of the hobby for me, so I would rather have a bunch of smaller (75-180) tanks than a couple huge ones that never change much. Also this would keep maintenance manageable, as I would rather still care for the smaller tanks myself than pay someone to care for all of them. It gives you a sense of pride when you've done all the maintenance for a fish and you watch them grow up in front of you.