Something in my fish tank

Question
We have a saltwater aquarium and have decided to turn it into a fresh water tank.  The tank has been neglected for over a year.  It had one damsel fish remaining.  I noticed within the last few weeks something that looks like insects.  Some are large, some are small, some are blackish in color and some are white or clear looking.  They almost look shrimp-like.  They sort of swim to the top of the tank, and then sort of float back down to the bottom.  They scurry along the sand on the bottom of the tank. Do you have any idea what these might be and how we can get rid of them.  Thank you for your help.  

Answer
Hi Trish,

    Most likely the little "insects" you are seeing are copepods or amphipods. They are part of the makeup of plankton, called zooplankton. A food source in saltwater habitats. The fish which thrive on these little morsels are mandarin fishes. If, however, you are turning this into a freshwater tank they will cease to exist.

                                good luck, gerry.