WHY I'VE STOPPED USING INSTANT OCEAN IN MY REEF

[David S]

Now that I had other reasons to go back to some old data I had, the lithium may well be coming from calcium additives:

Aquarium Chemistry: Purity Of Calcium Chloride ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/2004/3/chemistry

Lithium does not seem to pose as much of a toxicity concern as many other ions, but in three of the samples (Dow, Warner, ESV) it is greatly elevated. Over a year, each of these would add about 2 ppm lithium, or 12 times the natural level. Is that too much? I am not sure. In a prior analysis of artificial salt mixes, Craig Bingman found that two salt mixes started out with greatly elevated lithium levels (90X over natural levels for Coralife and 6X for Seachem, with the others ranging from 1.5X - 3.1X). Typical aquaria surveyed by Ron Shimek contained about 0.6 ppm of lithium (3X over natural seawater) with a range from 0.015 ppm (0.08X) - 7 ppm (39 X).

It is well known that excess lithium has significant adverse effects of the development of sea urchin embryos,1,2 and many studies have been carried out in this area. The amount of lithium used in those studies, however, is typically around 500-3,000 ppm. It has also been shown that 345 ppm of lithium will result in death of the isopod limnoria.3

So while 2 ppm lithium delivered by these samples is greatly increased over the natural levels of 0.18 ppm, it is still small compared to the hundreds of ppm required to show toxic effects. Given that gap, and the fact that the lithium levels will likely be attenuated by water changes, I conclude that the lithium in these calcium chloride samples is not an excessive risk. Nevertheless, that is something that each aquarist can decide for themselves.

So the Lithium may have been coming from my Seachem supplements.
As I no longer find the necessity to dose those products, I'll see if my LI levels stay comparatively low when I test with Triton sometime in the Spring of 2018.
While I expected a significant drop in my LI levels; like you I was a bit surprised that it went from 3500 ug/l to 240 ug/l in 2 months.
As you pointed out, I still had some "old" water in my tank (anywhere from 20 to 30%) so I anticipated a potential drop to ~1000 ug/l give or take a few hundred ug/l.