i would take your water to a lfs and have it tested for EVERYTHING (amm, nitrite/ate, phosphate, calcium, ... and whatever other test kits they might have). that might narrow it down. maybe too much calcium? maybe someone poured a glass of chlorinated tap water? (my dad did that to my fw tank a few years back) try to think of everything you can... did you add any chemicals that day, or the day before? i bet if you did, that could be the problem... well, i cant think of a whole lot more, i hope you find out what happend. good luck!