Pond clean out

Today we did a clean out on a pond that was built by the home owners friend. All in all not a bad job,but now 4 years later some things are showing up.We were told that the previous people that did the clean out some how killed all the fish after the fish were put back in the pond.A couple things come to mind first and the most common cause is dechlorination is always required on municipal water second and more specific to this pond, we found 8 inches plus of gravel on the bottom of this pond , it would be easy if the cleaning person didn’t know any better to decide that cleaning all the way down to the liner is not needed ( no one can see under the top layer of gravel anyway) this isn’t the case and is a sure way to get into problems. All that sludge and gunk under the gravel is one of the most natural toxins to your fish out there.When you re-fill the pond and don’t remove all that gunk it will well, or soak into the new water and probably poison the fish.