Thanks for your reply, @Seattle_Aquarist
I do weekly water changes of about 20-25%.
pH used to stay at 6.8, this week was at 6.6. Rotting plant leaves, I think.
I don't have a kit to test dKH or dGH. Is there one you recommend?
Is "Nitrates (ppm of NO3)" different than the nitrates number I posted of 10-20, sometimes up to 40 before a water change? If it's different, how do I test for that number?
I'm attempting to post four photos. The first plant is the one having the hardest time. I removed several all yellow leaves earlier today, before taking this photo. I'm embarrassed to say I don't remember what kind of plant it is -- it had been darker green and vibrant for a long time. (Java fern? Cryptocorynes?)
The grass-like plant is also looking less healthy than it used to.
The plant with the long thin leaves is looking pretty good.
The plant that I think is pennywort is hardly growing -- very tiny leaves, very little growth.
Sorry if my photos leave a lot to be desired!
Bump: Oh dear, I didn't realize it would post the photos sideways! They are oriented correctly when I open them on my computer!