Its a 6x1.5x2, so it won't fit in the tub. Good idea though
I thought about just spreading a huge piece of plastic out under it and raising the edges up to make a sort of bowl under the tank, but 125g is a LOT of water to try to contain indoors.
Its currently snowing, and sticking, so I may have to wait a couple of days till it warms up to move it outside and fill it. One of my friends has a tire business not far from here, maybe I'll see if I can set it up in one of there garages for a few days to test-fill it.