Another really good question. If you used the original sand or gravel or whatever you have then that is also I;d say the most likely source of ammonia. Moving a tank and not using new sand can muck up all the nasty crap in the sand and cause massive spikes. A good sized water change, like 50g would be a good place to start.
make sure the temp, salinity and ph are as close as you can get them so you dont shock what you have in the tank.