Make sure you are feeding enough. White poop can be a sign of insufficient feeding, but it can also be other things. They should have enough food to last about 5 minutes before being consumed.
TDS may provide a signal if there is a lot more in your water than the GH, KH and NO3 would indicate.
I don’t think that any of your GH, KH or NO3 readings are cause for concern, nor do I think that your lights are affecting the fish. This is something else.
has a good idea about using the 30-gal as a QT to rule out anything in your main tank. Leave the tank bare (no substrate) and no need to cycle the tank. Instead, put a bag of Seachem Purigen in the filter and monitor ammonia daily (Seachem also makes a good sensor; “Ammonia Alert” which is useful for this).
When I acclimate new fish, I match the GH of the tank water to that of the LFS water, before adding the fish. I also verify that the TDS of the tank QT water and the LFS water are within 10% or 50ppm of each other, whichever is greatest. After observing the fish for 3 weeks in the QT, I change about 25% of the QT water, daily, with the main tank water and, after four days of this, move the fish to the main tank. I keep Seachem ParaGuard handy in case a disease appears.
Prior to moving the fish into the QT, I keep the fish in a plastic trash can (dumping the entire contents of the LFS bags into the can) and drip acclimate over a two-hour period with water from the QT.
Don’t feed the fish until the second day and it is better to not use lights the first day.