soooooo.....how does a "crash" begin? i mean where could it start after cycling has taken place of course....i mean could it start with no parasite eating critters? and what else? and how does one protect from this? there has to be a plethora of reasons.
Jim, Crash is primarily a chemical phenomena. The best way to keep a crash at bay is ensuring that your chemistry is always in check and within the required range. If you are using automation to keep your chemistry in check then you need to ensure that you use high quality equipment that would not fail easily/with out warning. On the same note you need to maintain your equipment by doing preventive maintenance.
in my experience most crashes are due to user error or equipment failure. critters wont really cause a "crash" they will just eat your corals. To me a "crash" is ware your water parameters go way out of wack for one reason or another.
ways to prevent are to just keep a close eye on your tank. look for corals closing up or discoloring. Look for new algae growths or fish/inverts dieing. You can tell so much about your tank by just watching it.