Thanks for joining in, Mike. Since you are an old-timer (and have had reef tanks for longer than salsaking has been chopping tomatoes), what were some of the "plagues of the past". I have only been in the hobby for a bit over five years, and the major "issues" that I remember are:
trace elements (we don't know what they are, can't measure them, but our tanks will die if they aren't replenished!)
heavy metals (we think we know what they are, may or not be able to measure them, and their buildup are probably why tanks decline with time. Therefore, they must be removed!)
phosphates (we can measure them, but they may still be there even if the test says zero, so we must remove them to allow our corals to grow better and to prevent the growth of nuisance algae)
So, Mike, fill us in on some of the other plagues of the past. Hate to just get suckered by an old one, when there are lots of opportunities to spend money to prevent the new plagues!