I know these are hard to guess.. but they are claiming it's a 300g, I don't think it is. I'm thinking it looks closer to a 150g? It's been a while since I've looked to buy a new aquarium so my eye isn't as good as it used to be.
It very well could be a 300 gallon tank. That tank looks wide. If it is 96" X 30" X 24" then it is a 300 gallon tank. Does the tank look like it is 8' long?