I've been keeping hornwort from the same original plant for quite a few years, and I've always had the same problem: it goes along great, growing like mad and needing to be thinned out, and then it will "melt," which I'm not sure is the right term but seems to fit. The needles turn tan in color and fall off, and to say it makes a mess in the tank is an understatement! I always save a little sprig or two that remains green and eventually I get it going again, but I'm tired of dealing with its finicky nature.
This is a floating plant that does not have actual roots, and is a great plant for excess nitrates, so I like keeping it, especially for my more skittish fish that feel safer under floating plants.
I have read that temperature is the culprit, since the fact that it floats means it is closer to the light fixture, or just that it prefers cooler temperatures than many of our tropical tanks. Does anyone have experience with preventing the dreaded hornwort melt?
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