WoW!! Those are some High nitrates! I would not trust my test kit on that reading if I were you..
What fish do you hav in the tank.
On to the question:
First, you need to do some water changes to bring the trates down. Then you need to find out what is causing the spike. I suspsct that overfeeding and your undergravel filter are part of the problem. The problem with trates is the bacteria that converts them to free nitrogen is anaerobic. This bacteria only grows in no to very little oxygen areas. An UGF has movment and thus induses 02 into the bottom layer.
You have a wet/dry which is good for converting ammonia but does nothing for nitrates. In a FO system, nitrates are not as crucial as they are in a reef system. This does not mean that you should not worry.
My suugestions:
Water changes: regular water changes will reduce the amount of trates in your water. Use only bottled or RO water.
Find the problem: Either overfeding, overstocking or a combintion between that and your lack of anaerobic bacteria is the cause of the spike.
Fix the problem: Do not feed as much. Take a few fish out of the system to ease the load on your tank. Remove the UGF and put in a DSB or live rock. The live rock actually houses both aerobic and anaerobic bacteria. This, in my opinion, is the best choice.
Regular maintance: By doing regular water changes and cleaning your CC you can help remove some polutants in your tank. I belive this will be the most important step.
Good luck with your tank and be patient, it takes time to get it all right.
