Hornwort tends to get covered in calcium deposits, seen as whitish cuticles along the leaflets. This makes the plant less palatable to goldfish than most other 'oxygenators'. As noted above, they tend to fall apart in low light. Also, I wonder if your southern climate is a bit too warm for the plant?
John
Guess I'll have to look back into the temperature & light requirements before I order any more, LOL! If memory serves me (which it may not....), I think my water tops out in the low to mid-80's during the heat of summer. I have a thermometer probe at surface and on the bottom (at around 3-1/2'). Year round, the temps (above & below) only differ from each other by maybe 1/2 a degree, so the only difference the depth would make would be the amt. of sunlight. And right now, pond gets full sun for about 6-8 hours (split between an AM and a PM exposure with short periods of tree-shade). Sooo....
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Sorry to rant
, but I get frustrated
! These are the kinds of things you
don't learn about when you read about aquatic plants
. The ads or articles will say, "tolerates all light conditions," or, "thrives in lower light," or "does best close to the surface," or "does best at more depth," or "this plant will triple in size....." or, "prefers calm water," or, "likes to be in a current," or, "grows in all USDA zones....." And on and on. Pick 5 web sites, and you'll get 5 different descriptions about the
same plant!
So, most of the time it ends up being trial & error........
......'cause they don't go on to explain that there might be combinations of conditions for that plant to survive. Such as, "tolerates heat IF shade is provided," or, well you get the idea (I hope).