What size tank? 10 fish is a lot for saltwater; I don’t even have 10 fish in my 40 gallon.
Fish can easily survive nitrates of 40. Some people run their successful reefs at 50. The problem is…..
The rock! That’s what I was worried about when you said fake rock. The rock in the tank is the life support of the tank. It’s where all the bacteria live (ok, sand too) that process food, waste, and dead material then convert very toxic ammonia to nitrites and nitrates which are far less toxic in saltwater tanks. You probably did have an ammonia spike because fake rock just isn’t porous enough to provide enough living spaces for these bacteria. If you had had a well-cycled tank with lots of porous live rock I think your livestock would have survived the overfeeding most likely. It would have fouled the tank in other ways, but probably not killed everything overnight with ammonia.
Throw away the fake rock. Check out BRS or saltwateraquarium.com and look for either $$$ Saver Rock or Marco Rocks. Those I’ve found to be the cheapest options. If you have more money to spend there’s reef saver rock and rocks that have been seeded with dormant bacteria. In any case, cycle the tank really well, add fish really slowly weeks apart, and you’ll have much more success with this rock. I personally like cycling with Dr. Tim’s ammonia because you follow a recipe, test ammonia daily, and you can directly track the progress of your bio filter until it gets to zero before adding any livestock. One cycled, your bio filter will be much more bulletproof than fake rock.