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Discussion Starter #1
Is anyone using TDS readings to determine cycles of concentration vs city water for water changes? If so, what is your target cycles of concentration value? I have recently purchased a TDS pen for my hydroponics set up. I was tinkering with it and determined that I am running 1,000 PPM in my Planted Community Aquarium. My city water is 322 PPM, that gives me 3.41 cycles of concentration, pretty high is presume. I changed out about 4 gallons of tank water with fresh water and it is now down to 862 PPM. I plan to do another partial change daily until I get to somewhere between 1.5 and 2.0 cycles.

Thank you for any constructive criticism.
 

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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.
 

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my tap water TDS is between 50 and 60.
TDS in my tank is around 220. So we both have about 3 x tap water TDS in our tanks.
Rain water has TDS of 2 in my area, if I manage to collect some I usually add it to my tank. I only start collecting rain water after it has been raining for a while.
Keep in mind that adding salt will increase your TDS a lot.
 

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Discussion Starter #4
I plan to do partial changes daily until I get to somewhere between 1.5 and 2.0 cycles.
And so now after a month of 10% water changes I have lowered my TDS to maintain around 700ppm. With the make-up water it looks like the best I can do is +/-2 cycles.
 
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