Urea is pretty cheap
Just be careful with higher light and poor CO2/low pant biomass.
Low or varied CO2 will stunt tips.
You can see this in natural systems as the plant biomass and the competition for CO2 increases as the plants fill in during the growing season. Tips get progressively smaller, even though N and P are not limiting(plenty in the sediments).
This is why plants, particularly the weedy aquatics started to use bicarbonate. This is huge advantage.
How you dose at lower levels can influence CO2 demand a great deal in some species, but not others.
Not all plants have the same CO2 deamnd and ability to sequester CO2.
CO2 is 40-45% of the plant biomass as C.
So you will see the most dramatic issues, both algae and growth forms, with CO2 than anything else.
That's why 95% of the issues are CO2 related.
It changes fast, within 1 hour if can go from 30ppm to 3ppm.
Can N, P or any other nutrient change that fast?
Urea BTW rapidly changes to NO3 in water/aquatic systems.
Try NH4Cl, (NH4)2SO4 etc if you are interested in reduced forms of N.
Regards,
Tom barr