T5's throw light in all directions, not just straight down like we'd prefer. The reflectors only partially correct this. As a result, you get a difference between light intensity between the top and bottom of your tank.
Which can be a bit fidgety to work with. Lower the fixture too much, you might get algae at the top of the tank. Raise it too much, you lose red at the bottom of the tank, or further up.
All your plants, when originally planted, were cut short and therefore in the lower light portion of your tank. Now that your rotala is starting to get tall, you can see the red starting to appear in the new growth.
Your plants are growing, and (hopefully) your algae is minimal. That's the most important thing for now. As time goes on you can adjust the lighting to find what seems to be optimal for your setup. This is best done gradually, over weeks or months.
Slight nitrate limitation can also boost reds. If you're using the EI recommendations for a high-light and heavily planted tank, into what started as a lightly planted tank (I saw that thread and agree with the replies), then you might be around 50ppm nitrate. And even if you cut down the dosing based on the planting density, you're probably still not near limitation. If you want to experiment with this, I'd suggest getting a nitrate test kit. Remember that when you reduce KNO3 dosage below what EI suggests for your tank type, you're reducing potassium too, which you do NOT want to limit - so replace it with some K2SO4.
Finally, it can be very enlightening to contact the party you purchased the plants from. Find out what conditions the plants were originally grown in, and compare them to your own conditions. Sometimes it's helpful. Other times it's an expectation adjustment, if a spectacular plant was farmed outside in direct sunlight, emersed, drenched in fish-killing quantities of algaecide, and/or anything else you simply cannot replicate.