How are your water stats (ammonia, particularly)? Is your tank fully cycled (stable readings of 0 ammonia and 0 nitrite) or do you get readings regularly?
Regardless, water change and gravel cleaning are the first order of business. Get nitrite down under .25. Do a second test while you're doing that, I do occasionally get false positives, mostly with the ammonia test. If the tank was fully cycled, check around for dead fish, try to account for everybody. Test water after a couple hours to make sure it's staying down, possibly do an additional water change, then check every 12 hours and change water as necessary until it passes.
If you've been having readings, then it may just be a blip, which happens during and just after a cycle sometimes. The response is the same, water change and follow up testing, but chances are it's not something dead in the water.