to be honest ammonia would be the only one I worry much about. in all my fry/growout tanks I have an ammonia badge for watching the levels of ammonia, I test sg once a week but I do have ATO's on each tank and I watch my temp even though I have controllers for the tanks. other than those parameters I don't check anything else. weekly water changes on the growout tanks of 50% and daily 10-20% water changes on the fry tanks.
if you have no ammonia and your temp didn't jump real fast 5+ degrees or the salinity didn't make a huge jump then IMO that just leaves that the food was bad, no other parameters that we normally test for would kill off clowns that fast, but food poisoning could.