i see, so it's not some kind of deficiency?It looks like an older leaf that needs to be removed close to the crown. Don't wait until it looks awful, there's no point. There are a 2 small leaves that will need pruning soon too. 1 on the left with a browning curled tip & 1 on the right front that looks to be turning clear/transluscent.
what a beautiful tank. what size is it?Don't plant it in the substrate, hornwort will rot away from that.
If you want it "planted" take some fishing line and gently but firmly tie it around a rock or driftwood and use that to weigh it down.
Or secure it with a suction cup to the glass.
Or just let it float.
Mine in the right corner is secured with clip-type suction cups, holding the hornwort down so it grows upwards, but not planted under the sand so the base doesn't rot off.
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55 gallons, though it's been changed since that photo because of an accidentwhat a beautiful tank. what size is it?
Its looking more like a potassium deficiency, id dose liquid ferts, and check that it has potassium in it.