hornwort is a huge nutrient hog so ya it would be the first to go down hill from lack of nitrates. Weird your pothos are not growing roots. I can put a stem in the water and in 1 day see it growing out. It is possible its in shock, in the beginning I was lucky to get 1 leaf after 3 months. But now they grow great, dont give up in time you'll regret it because you will have a huge vine haha.