For the size tank you are looking at, the 2x4 rack you have there is sufficient. Additional bracing might give you peace of mind, but really isn't necessary. I would definitely not add any plywood on the top of the stand, as Brookster said the sides and back would be best. Hopefully your builder squared and leveled the 2x4 top - double check it with a level. provided there is no twist and it's flat from board to board, that is a much better surface than a piece of plywood is ever going to give you. Additionally because of its composition, plywood is very difficult to get a good seal on the edges of, which means it tends to absorb water and swell over time. Wrapping it with something will of course make it more aesthetically pleasing. Make sure you have sealed the 2x material, so that they don't absorb water and you shouldn't have any warping issues, provided your builder used good dry materials - if he didn't, wrapping it isn't going to stop it from twisting.