The parameters are ok in the thirtyeight now. I changed out 15 gal today (not the norm friday 20 gal wc because of so many small wc this week) and fed the plants usual ferts dose (still with skewed higher K2S04 and almost nil KN03 for nitrate levels).
The plants are far from happy, but I think I have figured something out (I know I can get this right because the tenner seems relatively balanced now, and my twenty was in a good state right before I moved and started messing with things). Reading on the forums, finally found a thread discussing the exact LED light I have- it's not a very popular one, turns out- and someone had taken the trouble to call the company and got the PAR values, so now I know for my tank height the light level is med. That's important to know! and a good thing too. I'm not just limited to low-light plants, as long as I don't crank things up high enough to need C02.
It's all the more confusing since the light strip didn't come with any instructions whatsoever. I had to figure it out from reviews and comments posted online (by other equally frustrated aquarists). So I looked at where I was before- when I first set up the light I had it on the longest fade in/out- 2 hours- and eight hours max of color-plus-brights. I'd reduced the colored channel and the fade in/out just for aesthetic reasons, because I didn't like the lights looking so yellow, but didn't stop to think what was best for the plants. I think that's when things started going downhill- looking back at my photos, it was doing okay here. Now it's not. Worse since readjusting after the trip.
So I've reset the lights once again. Considering the feedback I got on the forum, and where things were at with the first setup I had going on the timer. I put channel 1 (colors) to start at 11 am and run until 7pm, channel 2 (white) to overlap that, running from 1:30pm to 6:30pm, and the fade in/out to do 2 hours. So it fades in from 11am to 1pm, color only 1 to 1:30, color-plus-brights 1:30 to 6:30pm, color only again 6:30 to 7pm, fade out 7pm to 9pm, then blue/moonlight 9pm to midnight, darkness midnight to 11am. That's 8 hours total light, but 2 of that dimming in/out, another 1 hr color spectrum only, all of which I think isn't as prone to algae, and 5 hrs full lights. I'll see how that goes. It's more dimming that I had the second time I changed things, but less brights that the first time around. Remembering that when things were going badly on my prior tank, what fixed it was reducing the photoperiod.... The only way to know if this will work is to try it! Wait a week or two, take more pictures and judge by the plant response.
The other thing I can do is stuff more plants in there that consume the excess nutrients. With that in mind I added more watersprite and some ludwigia (I really like its reddish hues and purple undersides). I wanted to get hornwort (in spite of its reputation for shedding needles; it's supposed to be great for controlling algae outbreaks in a new setup) but I couldn't find it available locally.