Ceratophyllum demersum

They say that if there are problems in an aquarium, you could throw a Ceratophyllum demersum in there and everything would get better quickly. 

Perhaps, indeed, this might work in some cases. However, most often, it needs to be grown the same way as other plants. Another thing is that hornwarts suffer from a lack of light less often than other tall-stemmed plants. This is because they are floating macrophytes and their roots are more like an archaic part of their structure rather than a crucial factor in successful growth. Yes, if you have an average amount of light power, excess nutrients have accumulated and there are many fish that produce at least some carbon dioxide, then the hornwart can “eat” these nutrients from the water. However, these very same nutrients will then need additional monitoring to ensure they do not deplete and the hornwart has something to feed on. If the conditions in your aquarium are not suitable for him, which is not unusual, he may crumble into smaller components and scatter throughout the aquarium in green needles. The photo shows an example of what a hornwart might look like if it is kept as an ornamental plant rather than as a survival-orderly. By the way, after placing it in water, cyanobacteria can appear (and quite quickly), why do you think?