Can we also have the tank size? However I also recommend you pick up a test kit, and change the tank with treated (dechlorinated *not sure if that's a word*) water.
Let me jump on board. A minimum of a 50% water change no matter what you know or don't know. As soon as you can, get the test kit. It may be the first of many 50% changes in the next day or so if your tank was overfed.