Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Boron and Copper are probably the 2 items that will first become toxic. Plants require both, but cannot handle too much boron, and shrimp and snails are not tolerant of copper.
How much it takes to be toxic will vary with your system.
in a high tech system the plants might be so active they will remove a lot more trace minerals than they actually need, providing a big cushion.
On the other hand, if your tap water is acidic, and you have copper pipes, you might already be really close to copper toxicity, and even a regular dose of trace minerals will be too much for the more delicate livestock.