Most def. dont nail it to the floor… id suggest using solid cinders to keep it off the floor. I use them to raise my tank stands even in my basement fish room for the same purpose. There plenty strong to hold that entire frame up. The 4” is enough to let everything dry out if/when water is spilled. You can also use just 1 layer of 2x4 or 4x4. Any bowing will occur at the top of the frame, not the bottom. What the center brace is for. Aside from normal “fish water” spilling or a potential leak that could need repairing, water from ur vehicle will be brought it when it rains or snows. That alone can easily rot out ur 2x4 frame if it can’t dry out. Ill add a cple pics of 2 ways i raise my tank stands off the floor. Again you dont “have” to raise the frame up, its just a good idea to keep it in good condition. Highly suggest using a good oil based paint on the bare 2x4 at the least.
