1000w is better suited to a space at least 4x4 feet- spend ALL the money you saved on plastic on a high quality, high CFM blower!!!

To keep the tub properly drained, tip it so all the water flows to one corner.

I'm not all that hot on the gravel idea. Risk of interactions between minerals in the stone and your acidic solution is high- especially if the gravel contains bits of limestone, marble, or 'puddinstone' or another conglomerate that may contain bits of these. You could find yourself with rising pH that is difficult to maintain. Try hydroton instead- after initial treatment (just a few days' soak in a solution adjusted to pH 6 and maintained there), it's inert, plus it's light and porous and maintains an ideal moisture level. I'm all about CHEAP, but choose your substrate carefully. To save money and weight, consider doing containers (6" or 8" polypots) of hydroton. Then you don't need as much of the stuff.

Have fun!