If you are growing red (not green) Rotala rotundifolia (or its red relatives: H'ra / Ceylon / colorata...) and wonder why it doesn't look as red what you see in pictures, this may be the reason. The picture above shows the exact transition when we switched from a 'rich' fertiliser to a leaner one.
First off, let's assume good overall health: no algae, new leaves are denser / bigger / more lush than older leaves. Good health is the basis of good colour. While Rotala can grow in tanks without CO2, it takes alot more skill to have it healthy in CO2-limited environments.
Secondly let's assume your light is strong enough. You definitely need medium lighting (60 umols of PAR) and above to bring out the colour.
The last aspect is the tricky part. Rotala rotundofolia needs well rounded nutrition to be healthy. However to become really red, the nitrates in the water should be close to zero. One of those paradoxical 'look before you leap / he who hesitates is lost' conundrums.
Above, we took a portion of Rotala rotundifolia originally grown using APT Complete (on the right) and grew it using APT Zero (on the left) which has zero nitrates. While both are 'red', the plant on the left is clearly 'pink-red' while the one on the right is more 'orange'.
Since less is more, can I simply go without ferts?
No. Rotala rotundifolia requires a complete, nutritious diet to be healthy.
So, just remove all nitrates?
No. The total absence of nitrates would result in stunting and poor health (which makes it vulnerable to algae etc...)
The technique is to use fresh aquasoil(different from inert and synthetically enriched inert substrates), while dosing a fertiliser that has some, but low enough nitrates to bring out the colour. Aquasoil serves a store of nutrients, making some nitrogen always available through the roots. When combined with a leaner fertiliser, you hit the sweet spot.
The stored nitrates in fresh aquasoil become depleted after around 3 months. At that point, you can either add fresh aquasoil, or enrich the substrate with a root fertiliser such as APT Jazz.
So I should NOT use APT Estimative Index?
Exactly. The Estimative Index approach works for plants that thrive in nitrate-rich waters. With APT EI, your red Rotala will most likely be healthy, but not red.
So I should use APT Complete?
Probably. If you have a heavily planted tank, the resultant level of nitrates using APT Complete is generally low enough to induce 'nitrate limitation' (think of it as a Ketogenic Diet of sorts) to bring out orange-red tones you see above. But if you have a small or less heavily planted tank, then the nitrate level may still be a little too high. Try lowering the dosage, or use APT Zero.
So I should use APT Zero?
Very likely. When paired with fresh aquasoil, or old aquasoil (> 3 months) that has been enriched using APT Jazz, APT Zero delivers the 'sweet spot' combination of complete nutrition except nitrates and phosphates (available in aquasoil, and in fish waste). At these levels, you get the reddest possible outcome.
Is there a catch?
The nutrients in new aquasoil get depleted over 3- 6 months. It is best to use APT Zero combined with APT Jazz.
Another fine point is that light strength matters, but so does spectrum. To bring out the tones you see below, you need either a T5 array with stronger Red and Blue tubes, or LEDs that have have sufficient red/ blue diodes to bring out the red tones.
Above: A tank with a variety of red plants using APT Zero in the water and APT Jazz in the substrate.