Plants need nutrients,
CO2 and light and will let you know if there is anything lacking. One of these 3 will be the limiting factor to a plants growth, so increasing any of the other 2 without increasing the one that is limiting will achieve nothing but algae.
In most aquariums with low demand plants the limiting factor will be light.
If you arent injecting
CO2 then
CO2 will get into the water from a variety of routes. Fish respiration is one. When plants arent photosynthesing they take in
O2 and expire
CO2. Gas exchange will occur at the surface and as
CO2 is taken up by plants it will be replenished from the atmosphere, so an airstone or filter that gives good surface agitation will promote this gas exchange. Many of these low demand plants use carbonate hardness from the water and convert it to
CO2 in the leaves. This is part of the process of plants transitioning from emersed to submerged growth as the plant adapts to use
KH rather than atmospheric
CO2.
2 essential nutrients that are provided by fish are nitrogen and phosphates. These can be supplimented with fertiliser, but most fertilisers have essentially zero nitrogen or phosphate because they are seen as promoting algae growth and are normally supplied by fish. In planted tanks that are lacking sufficient fish, or where they are very heavily planted and just use up more than the fish supply, there are fertilisers that do have a good amount of nitrogen and phosphate in them.
NA Thrive is a good one to go for if its available in your location (America). We have TNC Complete here in the UK. Aquarium Coop make one and i think tropica do too. Aquarium Coop recommend 50ppm nitrate for healthy plant growth which i think is a bit extreme, but you do want some nitrate in a planted tank which these fertilisers will supply where fish dont.
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