Hornwort question

Let me start off by saying that I keep large bunches of hornwort without any real problems and go on from there... The conditions you describe (fert. & light) are by far better than mine and would suit most plants, so no worries there... Now if you really don't want any problems with hornwort, you should let it float near the water's surface as it is a floating plant, this way it is going to get plenty of light and CO2, but it can also be kept submerged (in my case) with limited problems: 1) If you bury it in the substrate, the end is going to rot eventually and it will float to the top, so you'll have to replant it once in a while (personal experience), what I haven't tried is to attach it to a piece of rock or driftwood, I suppose that way the end will get enough light and will not rot away... 2) Eventually some branches will turn brown and the "needles" are going to fall off, because the plant grows fast and shades itself to the point where some braches do not receive light (floating will prevent this). 3) Sometimes the "needles" do start falling of, but that only means that the plant doesn't like the conditions it is kept in, if the conditions are good it will not do that.

Worst case: if the plant does shed its leaves, simple gravel vaccuming and filter cleaning would fix everything (you have do that once in while anyway (in most cases) )...

I'd say that you never know what plant is going to thrive in your tank unless you try it out... Hope this helps.

The needles/leaves fall off if the plant isn't getting the right nutrients. So I'm not suprised that the pet store people are having problems. In a nutrient rich enfironment, It doesn clog the filters. It does grow fast, and I was taking handfulls out every week and throwing away just so it wouldn't overtake the tank.
If you over feed or over fertilize..it takes up the excess nutrients. I got tired of mine and got rid ot it....I had a big algae bloom which I'm still fighting.

It can provide too much shade for plants lower in the tank...just tear off what you don't want and pitch, or give as presents to your fish keeping friends
I did loop mine around some driftwood, but don't bury it like a rooted plant...it's not.