Since you don't have many fish, I would recommend that fertiliser if you want it to grow fast (which will help it clean the water too and ward off any algae), yes
.
But make sue you don't get another brand as then the hornwort will likely be limited by nitrogen and possibly even phosphate.
If you cant get that brand, I would recommend preferably Seachem Flourish or any other cheap brand in addition to Seachem Nitrogen.
The reason behind this is a lot of fertilisers (such as the API, Kent, Tetra and Nutrafin brands) are designed without some of the nutrients that plants require, because old heavily stocked fish fish tanks that don't get many water changes already have these nutrients, and when people add more it sometimes causes (or is at least blamed for) algae.
So in your tank, which is lightly stocked and probably gets the right amount of water changes, the plants will be lacking in these main nutrients (usually nitrogen and phosphate) so using a fertiliser without them would basically be a waste of money.
Tropica Plant Nutrition + is one of the few good ones that contain everything for the plant, but it's not often seen in LFS's so you may want to get it from the internet.
If your interested enough, you can make your own fertiliser that's just as good, but more flexible and cheaper from buying powders and mixing them up yourself, but it requires a bit of reading (look up EI dosing if interested).
Seachem produce good fertilisers, but their broad-ranged Flourish fertiliser doesn't contain enough of all the nutrients, so you will have to supplement it with Seachem Nitrogen and maybe also Seachem Phosphorus /Potassium if you get deficiencies, so this is an expensive way of doing it.
If you want it to grow slowly,your better not adding anything at all than a bad fertiliser
.
