There are a few things you need to take into consideration before you attempt this idea.
First, yes, a piano is in wheels. But even a grand piano only weighs in at about 1,000 lbs. Your looking at over 1/2 ton in water alone if you have a sump. Add in at least 200 lbs of rock and sand, the stand and all your equipment. Now the needles pointing at close to 1 ton (2,000 lbs). If the tank isn't sitting on a concrete slab, you'll be supporting the floor underneath to carry all this weight. If you move the tank for any reason, your moving it off that support and onto unsupported floor. Not a good idea. Even if you move it 6 inches, your compromising the integrity of that support.
Second, a good tank stand will have full contact with the floor all around the bottom to evenly distribute the weight of the tank. If your husband does build the stand, he should make the top and bottom framing out of doubled up 2x6's for strength and install at least 6 of the largest casters he can find and double up the support on those as well.
Third, Water is not a solid. It wants to move, even after the container it's in stops moving, the water wants to keep going. If you've ever carried a bowl of water, soup or even a glass of milk that's full to the top, you'll know what I'm talking about. Not only does the liquid shift, the weight of that liquid does also. A tank on a stand also raises the tipping point creating a higher center of gravity making it top heavy. Try carrying a glass of wine by resting it in the palm of your hand and then stop dead in your tracks while trying to keep the glass or bowl balanced.
Last but not least, do not attempt to do this on carpet. No matter how big the wheels are, they will dig into the carpet and you might as well be trying to push your tank up a hill.
I'm sorry to say this but the odds of success in the long run are not in your favor.
But that's my .02
