Well, there's a couple of potential sources I can see so far...
You're using city water, as in right from the tap? That could be a source of nitrates (among other things). You should consider investing in an RO unit. Pure source water is going to make your life as a reefkeeper MUCH easier.
Do you have some sort of plastic biobale, bioballs, etc. in the wet/dry filter, or are you just using it for a sump? If you still have the plastic media in there, that could be another part of the problem - that and the built-in skimmer.
The problem with the plastic media in a wet/dry is that it harbors lots of aerobic (nitrifying) bacteria, but no aneraerobic (dentrifying) bacteria. So, it doesn't really come full circle with the filtration and leaves you with an excess of nitrates.
Also, generally the skimmers that are built-in to the wet/dry units don't work very well. A high performance protein skimmer can help eliminate high nitrates indirectly by removing organics before they break down into ammonia, then nitrite, then nitrate.
Test your source water first. If you don't have any nitrates in it, I would next look to the wet/dry filter media and lack of skimming efficieny...
Also, what kind of trigger do you have? Some are not reef safe...