Please don't take offense at the question. You did not state in your post all the information about your system. Generally whenever posting a request for help you should supply as much information about your system as possible. Please post actual number for all water parameters. You would not beleive what some people think are 'normal' or 'spot on'. Here is the list of questions I normally ask:
How many gallons is the tank?
How long has it been up and running with fish?
How and when did you cycle the tank?
What type of filter/filtration are you using?
Describe the lighting units (type bulbs, total wattage).
What are the current water parameters? Ammonia, nitirte, nitrate, pH, salinity, temp, calcium and alkalinity. Please post numbers of results.
What is your source water for top off and partial water changes?
Now I have more questions for you.
What is the make/model of the skimmer you added?
Did you clean the skimmer before installing it in the system?
Sometimes there are oils and/or other contaminents that occur during the manufacturing process or while it is being shipped or sitting in stock somewhere. These may not show up in the normal water tests we run but could cause stress to the tanks livestock. Whenever in doubt I start with a 20-25% PWC
till I can figure things out.