Here is a excerpt from Randy's article. The full article can be found
here, Randy's Article.
Elevated Boron: Toxicity
At boron levels above that present in natural seawater, as is supplied in some artificial salt mixes and as may develop from overuse of boron supplements, boron begins to exert undesirable toxicity on a number of organisms. The studies on marine organisms are not wide ranging, so one must be careful in how to interpret levels above natural seawater since tests have not been run on most of the organisms that we keep.
In general, marine organisms (invertebrates and fish) are seemingly more prone to experience toxicity from boron than are freshwater species. The marine isopod Limnoria lignorum has a 24-hour LC50 (that is, the concentration at which 50% die in 24 hours) of only 2.6 mM (28 ppm boron).16 That is only about 6 times the concentration in natural seawater (and is BELOW the concentration in Seachem salt mix!). Similarly, the dab, Limanda limanda (a North Sea Fish), has a 96-hour LC50 of 6.8 mM (74 ppm boron).16
A lot of additional biological effects can be found on the web sites of the Canadian Environmental Protection Division Ministry of Water, Lands and Air Protection17 and the United Nations International Program on Chemical Safety. 18
Elevated Boron: Confounding Interpretation of Alkalinity Tests
One additional complication that comes from substantially elevated borate is the confounding of the interpretation of alkalinity tests. 19 When reef aquarists are concerned about alkalinity, they are almost invariably concerned with the alkalinity that comes from bicarbonate and carbonate, and it is largely used as a surrogate measure of bicarbonate, which is necessary for calcification.20 Nearly all hobby test kits measure alkalinity with a single titration that provides total alkalinity,19 which is the sum of bicarbonate, carbonate, and borate alkalinity. When the levels of boron are similar to natural levels, then the contribution of borate to that test is minimal, and is generally safely ignored in guidelines for alkalinity (for example, keeping a reef tank at 2.5-4 meq/L total alkalinity).
However, if the boron level is substantially above natural levels, as it is in the Seachem salt mix with 12x normal levels, borate can actually begin to dominate such tests, 19 and makes knowing the real bicarbonate and carbonate alkalinity much more difficult. Seachem sells a special borate alkalinity test kit to try to disentangle these effects, but that is only really necessary with tank water that contains greatly elevated boron levels.