The length would be fine but I would be a little concerned with the width, since bonnets can reach 4-5 feet (in captivity you can reasonably expect it to reach just over three feet). As long as you have a home for the shark when it grows, it sounds like you have a pretty decent system. As to substrate unless you intend to have some bottom dwellers I would suggest going with a thin (>1 inch) layer of a fine coral sand and would avoid silica sand (slilica can sometimes cause irritation to a shark's ventral side). Bonnets are open water sharks therefore they will not spend any time on the bottom. The only time my guy is near the bottom is to pick up any food it didn't catch in the water cloumn. I would also be very cautious about what animals I keep with the shark. Even though bonnets are open water sharks they can still be pestered by large angels and more agressive marine fish.
It is highly recommended that you feed a varied diet and use vitamin supplements. I feed my sharks ( I also have an Atlantic Sharpnose) a combination of squid, smelt, mackerel, blue crabs, live ghost shrimp, table shrimp, and fresh whiting. I use Kent Zoe and Selcon as my supplements to the food. I also periodically will feed the sharks iodine tablets from Purina (this aids in avoiding goiter disease-a deadly possibility). On occasion I will use other fresh seafood, depending on what the fish monger has available.
Once again good luck and let me know how things go for you.
