Maybe I need to clarify. I have a friend that has low light tanks with plants in almost all of his tanks. He doesn't use much in the way of fertilizer, if any. His plants grow slow, which is to be suspected. They look awesome despite no CO2 and such. I'm still quite new to plants myself and thought I'd ask you guys for enlightenment. He was asking about liquid fertilizer to get his plants to grow a bit quicker without going the CO2 route. I've been getting good results so far with regular dosing of Flourish Excel and occasional Flourish original. I do have better lighting than him, although nothing impressive.
I'm wondering if he doesn't increase his lighting but starts adding fertilizer of some sort if his plants will start having problems. If I were to take a guess I'd guess algae problems would happen. He has many tanks, and has for many years. He doesn't have problems with algae now, and I'm sure its because he keeps up on his water changes.
Would the plants be unable to use those extra nutrients and end up being wasted in water changes? I wouldn't know how to tell what kind of adjustments would be needed according to the lighting because I'm too new to this, as is my friend. How do you know? There's so much information here I get a bit lost trying to find answers. I link would be helpful.
It sounds to me that your friend's tank is doing well just the way he is doing things. If it's not broke don't fix it. Like you said the plants are expected to grow slow, since he has a low light/low tech setup. If he want's the plants to grow faster then doing a combo of things like increasing light, ferts & adding C02 is the way to go. Adding ferts alone will not get him the results he is looking for, but may get him some he does not want.