That was very helpful and informativeThis takes some explanation.
Plants grow by photosynthesizing. In order to photosynthesize, there has to be sufficient light (intensity and spectrum factor in) along with the necessary 17 nutrients in sufficient quantity to balance the light. And each species of plant has its own specific requirement for this balance; fast growing plants require more, slow-growing plants less, generally.
Light is thus the most crucial factor, as it must be of a certain intensity and spectrum to drive photosynthesis. Duration does not compensate for inadequate intensity/spectrum. So assuming the light is "OK," we can consider the nutrients which is the substance of your question.
Fish excrete all the nutrients plants need. There is also the breakdown of organics primarily in the substrate which adds more (especially the CO2). Water changes replenish the minerals (except obviously in very soft water) as well as removing substances that are detrimental to plants and others detrimental to fish. Depending upon the plant species and numbers, the light, and the fish load, some plants will manage very well just from the aforementioned.
If it is necessary to supplement any of these nutrients, it is important to do so with a balanced "fertilizer." An excess of some nutrients can cause plants to shut down assimilation of certain other nutrients; the excess can also feed algae because algae is not as fussy as higher plants over light and balanced nutrients. This is why comprehensive supplements are best; they provide what the plants need as a supplement to what will naturally occur in the aquarium, but the nutrients are in specific proportions to each other. Algae is much less likely to take advantage, because the plants are able to use what is being provided...again assuming everything is basically in balance.
Some plants benefit from substrate supplementation with products like Flourish Tabs as opposed to a liquid fertilizer. The real benefit here is that the plants can utilize the nutrients as needed while the nutrients are not just released into the water where algae may use them.
Comprehensive supplements include Seachem's Flourish Comprehensive Supplement for the Planted Aquarium, and Brightwell Aquatics' FlorinMulti. These are two I have looked into and used, but I'm sure they may be others. However, do not resort to individual nutrients; usually this is not going to benefit because it throws the balance out. The complete supplemental products contain what is needed in proportion as needed by plants, so unless you overdose (or under-dose), algae is not going to benefit.
I really like Colin's suggestion for when you don't use fertilizer. I have never had anything other than low light plants and following that advice have had my plants do reasonably well. Got to watch room light from windows though. You will see a couple pictures on these forums where algae really took over.If you use an iron based fertiliser, you can monitor the iron level with an iron test kit and keep the iron level at 1mg/l (1ppm). If the iron level drops bellows 1ppm, you add a bit more fertiliser to bring the level back up to 1.
If you don't add fertiliser, then just feed the fish and give the plants some light and don't worry about it unless you get heaps of algae.
This could work in some situations, and I do not doubt you that it seems to be for you, but it is a dangerous method and frequently not successful. I have killed my floating plants by simply dosing too much iron. I did the experiment twice so I know it was the iron.I found Iron to be near a cure all for plants. I don't fertilize at all but I'm getting deep green lush growth when before it was just so- so to poor. I couldn't figure out why. I had the lights,the filter..but plants were not growing well..and I was getting far too much hair algae...just choking amounts in the 240 gallon that got window light besides over head.
I made the filter bigger..that helped. I covered the fine sands with normal sized aquarium gravels...that helped kill any residual blue green algae off. But what really did in bad algae was when I tried Seachem iron..all my plants flushed a deep green..plants that had died down to the rhizome have started to regrow. My Sword plant has just taken off fierce!..after a year of grow,dieback,grow,dieback. A Bolbitis heudelotii that I thought was a black goner...is sloooooowly reviving. My tap water must have been totally devoid of iron. I've dosed what the bottle says PER WEEK..since I feel like I'm feeding starving populations ...I might cut in half from this point..I have to just watch for results on that.
Its like I hooked up a Co2 system. Yet,all I Do is toss in capfuls per week. I know that since I've gotten great results...there is a much less costly iron for hydroponic and aquaponic sold at the Hydroponic stores ( or internet)..I have to try that next.