Hornwort can appear dead when it lays at the bottom of pond during the winter, but it is not. People often accidentally throw it out when they net out the sludge in the early spring too, as it can be hiding the sludge too. The fish also love hornwort and can nibble it slowly away over the spring before it has a chance to get big. The trick is to have extra stashed away in a fish tank or jar of water in the window until you have a good size reserve clump saved. Eventually you will have a large enough clump that will make to spring without the fish eating it all.
If you look close at my picture of the bottom pond, you can see the thick mat of hornwort that looks right now like algae, but that clump will change quick when the water warms up and look much better.
Most of the fish are hiding right now in the hornwort in that pond.
The second picture you can see reserve of hornwort growing right now in my old fish tank in my greenhouse too. I won't be needing that reserve this year as I have now way too much and will give some to my favorite fish store guy for him to sell.
The last pictures are from last fall, you can see the very large (20 plus inch) Koi from my top pond swimming in the thick hornwort.