Here is another thought. When I built my deck with a hot tub on it, 480 gallon tub, plus the shell, filters, pumps, and 6-8 people. I went to Lowes and they could tell me what size of boards to use. I would think they could do the same for a fish tank. Give them the size of the tank(not gallons, but dimensions) and the weight and they could tell you what kind of support you need, then check this back to your house.
The adding of OSB boards to the floor joists makes the most sense to me. If you wanted could you also add another 2x4 to the 2x8 to make it a 2x12 and add the OSB to this. Would that make it stronger? Just a thought, i have no idea if that would work or not.