Nitrates

Question
I work at a pet store and we have been having difficulties keeping our Nitrates down.  The total gallons for water is 1196G. We have a very large sump connected to the tanks.
We did a 40% water change 2 days ago and yesterday everything tested out fine, today its back to 110 .  We use Prime for our water conditioner and have filter media in the sump that is suppose to help with Nitrates... what can we do to fix it and keep it good .
Thanks
Look forward to hearing your response.

Answer
Kerri,

 What type of fish are in the tank and how many are in it? The best way to keep the nitrates down is to make sure the tank is not over populated. Regular water changes will help dramatically. I know at a busy store, the water changes don't happen as often as they should but if the tank has a lot of fish in it along with plants and other life, the changes really should be done weekly. Does the tank have an under gravel filter, they contribute a lot to nitrates. If the nitrates are that high, the ammonia and nitrites have got to be up too. Just do not overstock the tank and don't over feed the fish and make sure you are doing weekly water changes.