Brian wrote:
Can DOC (Dissolved Organic Compound) be measured in a tank?

You need a chemical hardness test kit and a conductivity meter. DOCs
are not detected by a hardness test, but they will measure on a
conductivity meter. A conductivity meter measures TDS (Total Dissolved
solids) - both hardness and DOCs. The difference between the two test
determine DOC. If you put off water changes (long enough), two things
can happen when you do get around to the water change.
#1 - Old Tank Syndrome - water change leaves the fish showing
immediate distress, gasping, lose balance, lay on side and die. The
cause - aged water slowly consumed carbonate buffer (low kH), and the
pH slowly drops. Toxic ammonia becomes non-toxic (ammonium) and builds
up. Then, with a water change, the buffer is suddenly restored.
Non-toxic ammonium reconverts back to it's toxic form ammonia, and the
fish die of ammonia poisoning...
#2 - Most likely to happen is Osmotic Pressure Shock. Without weekly
water changes, just topping the tank off, TDS levels get high. A water
changes lowers TDS too quickly and the fish suffer from osmotic
pressure shock. Osmotic shock interrupts healthy gill funtion by
reducing the intake of oxygen and releace of carbon dioxide and ammonia
from the fish. This can harm or even destroy the fishs
gills............... Frank (FB)