I used some generic make your water safe additive - let me see if I can
find
it.....it's a small bottle labeled PRIME. Detoxifies chlorine,
chloramine,
nitrates, nitrites. I have no real idea what is inside of it. What is a
good
water treatment to make the water safe?

What are your light levels?

I added some HW to my tank a few weeks ago and it has not done well. The
ends of the plant were turning brown and the needles were falling off the
entire plant. I added some Potassium to the tank (advice here) and it is
doing better, but not thriving. I suspect my light may be too low (1W/gal)
or it is missing some other nutrient. I doubt the Prime is your problem.

Nitrates are not a problem for me, as the GoldFish provide plenty.



I have a 15w fluorescent light, the standard 10 gallon top. I don't think it
is an expensive plant bulb, just whatever cheap bulb it comes with. I've
grown hornwort before, and it usually grows like the weed it is. I also
stuck a bunch of frill in the tank, and 3-4 days later it is very noticeably
starting to take off. The hornwort that didn't die shows signs of growing,
but it's too early to tell. It didn't completely die, half of it is still
alive. I wonder if this batch came from a very different environment, and
just doesn't like my tank?