Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Many people do use the TDS to determine water changes, but as you suggest not letting it get that high between water changes.
Fish do not want a sudden, sharp drop in TDS. Better to do smaller water changes that drop the TDS back to the target over several weeks or a month.
I run most of my fresh water tanks between the high 200s (tap water is usually around 280) to about 500. 500 only when I cannot do water changes for a while, with tap water top offs. My preferred parameters are to top off with rain water, and not let the TDS go much over the high 300s.
I also use TDS when I buy new fish. Make Q-tank parameters match the conditions in the bag- GH, KH and TDS. Then change the water several times while the fish are in quarantine until the Q-tank water matches the display tank. I have had problems with delicate fish (Hemiodus especially) if the water change results in a drop in these values of more than 10% in any one water change, or any more frequently than twice a week. Most species are more durable than this, but I know this is a safe value, so I usually target that much change as a maximum, no matter what species.