If you all pick up an iddue of AFQ from a couple of issues ago, you will
find what I wrote about humic acids in marine/reef systems, some of which
will be partially appicable to your needs.
Colorimetric tests for humic acids come in two flavors. Those that
measure the amount of yellow color in the water, and those that measure
the amount of reactive phenol functional groups in the water. Hach and
LaMotte sell test kits in both flavors.
I would imagine that the "measure yellow color" approach might be more
practical for freshwater systems, as it is for reefs. Both the LaMotte
and Hach test kits are described in my column. The LaMotte kit is about
2x as precise and 2x as sensitive as the Hach kit.
There are a number of chemistries for measuring the phenol content of
water. Almost all of them work at high pH, and there is a big problem
with magnesium crashing out of solution when any of these chemistries are
run in a marine matrix. This should not be a problem for most of you in
your freshwater tanks. Yes, you will be able to measure humic acids
colorimetrically in most cases. At least if your aquaria are typical.
What is the optimal concentration of humic acids for freshwater systesm?
That is an open question, and your data can help answer that question.