As someone who has over 800 gallons in aquariums between three tanks in his house, I'll say structural support is not as important as people make it out to be (ask an actual structural enginer). Anything up to 120 or so is just fine hanging out on a floor without extra support.

Also steel is completely unnecessary unless you're doing something wild like that guy in NYC with a 17,000 gallon home aquarium. If you're just throwing a 200 gallon or so on a second floor, an enginer might not care at all if its across the joists or has a wall below it, but even if its completely open below, the "support" might just be a joist of a different design, or an inch thicker or one extra joist. Steel is really not nessecary