Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you want to keep on trying to figure it out here is what I would do:
Set up several jars with the same water you use for the tank. (tap, RO, blended...whatever)
First jar: No additives
Second jar: Whatever you normally add to the water: Dechlor, minerals... anything you usually add when you are prepping the water for a water change.
Third jar: handful of whatever substrate you are using. If you have 2 materials like cap and substrate use separate jars.
Fourth jar: Samples of whatever rock you have in the tank. If you have some chips that came off the actual rock, that is OK, or test the actual rock (maybe in a bucket, if the rock is large)
Fifth jar: Start with the same water that you used in the tank (repeat jar 2) and add fertilizer as close as possible to how you dose the tank.
Sixth jar: Sample of anything else in the tank.
Test all these for GH, KH, pH and TDS every few days. Run the test out for a week, and do a 50% water change on all the jars that have shown any change.
How to interpret the results: If only one jar raises the TDS during the test, then that is the thing that is contributing. If that thing has more than one component (especially jars 2 and 5) you might want to break down those into individual tests.
Example: If jar 5 showed the TDS and GH rising, then you might look at your fertilizers. Are you adding GH booster? Any source of Ca or Mg that would do this? If you cannot figure out which fertilizer might be doing this, then set up one jar for each fertilizer and run those separately.