Perhaps the plants are rooting and will start growing better in a bit.
Is it the new Crypt leaves that are brown around the edges? Old leaves generally do poorly in a new tank and don't live long.
My S. repens has been in this tank for maybe 6 months. Was uprooted and replanted a month ago and just this past week perked up again.
Zero KH and GH is not a good thing. Plants need the calcium and magnesium in GH. The carbonate in KH helps with the CO2 I think.
My tank water tested at more than 40 whatever the unit is nitrate. I stopped dosing nitrate and the plants are showing color. They are not lacking nitrate, it is just low enough the plants show color is all.