Your tank has basically cycled, once ammonia and nitrite are undectectable it's done. With over 50% of the rock being pre-cured, a decent amount of nitrifying bacteria would have been added. It would as seaham358
pointed out, greatly speed up the cycle time. It by no means indicates the tank is stable though. On the whole, the level in which the nitrogens spike up to is not really that important as long as they have been detected. It does depend on seeding source, amount and overall surface area's though.
If you still have a decomposing shrimp in there and your getting these kinds of readings, I'd say it's all good. If your ammonia source is gone, keep feeding the tank as if there where fish in it to maintain the food for the bacteria. This will keep the bacteria in higher number for when waste producing animals are added. Bacteria grow/multiply to meet the demands of the tank through the wastes produced. If nothing is decaying and creating ammonia or through fish respiration, then the bacteria will decrease.
As far as your lights are concerned, 2x96w on a 40 gal
should do quite well. How deep is the tank?