Testing the water now will tell nothing about what might have been the cause. For to be able to start a trouble report one should have the availability over parameters in the past and a log book of what happened in the tank. In most cases problems are related to things one can not measure but sudden changes of often measured parameters may be the messenger something is wrong. What I advice is: when often measured parameters rise suddenly, this when no visual problems may be detected, first find out why, if there is a connection with limited availability of other essential building materials which are not measured , can not be measured or show 0. This can be done by adding a bit of modified f2 media, modified for the increasing messenger, which often is phosphate and or nitrate, and wait and see what happens. Silencing the skimmer also may help. Correcting the messengers, the increasing level(s), without knowing there is a connection with building material availability may cause what is shown here, depending of the methods used to control or lower the levels. A high nitrogen availability ( which is not the same as a high nitrate level) in combination with insufficient essential building material supply ( often phosphorus in LNS ) may mess up de coral bolobiont in a way it may not be able to find its balance, causing symbiodinium loss, called bleaching. When organisms show a high growth rate ( increased temp?) and suddenly on or more essentials are missing, they will grow to dead. In situations where one organism depends of an other it may be fatal for both.