Iron supplements may help with Downoi I've found. I don't have Co2 in my tank with downoi, with only medium lighting. Which may be why it hasn't really grown at all, but has looked nice and healthy for 6+ months now. For the 1st month I kind of doubted it would make it, thinking it was my set up. Once I started dosing seachem flourish iron it, kind of sprung up. Not sure if that's what helped it, or if it was just coincidence, but I read Iron is needed for downoi. I'll put it in my new set up once the co2 is hooked up, see if it helps the growth at all.
Ph is 7.6-7.8 in that tank