So prolly its from uneaten food which splitted at feeding time , it cant be anything else. I can before 2-3 days fungus on poops but i removed it and thats all. Try to remove it with your cleaning pipe which makes water changes. I am feeding mealworms and krikets one by one and removing after feeding every small piece of uneaten food. I ll be hard to remove some of them in your tank because of plants and wood but if you see any try to. Well if thinks go worst you may suffer some bacterial infections like your first tank....I suggest to have near a Nova+ which is natural prodact ( i use the same) in case fungus appears. I hope it won't