Some plants need nitrogen deficiency to turn shades of red not just iron. Obviously trying to limit your N can be detrimental to other plants as well as the plants your trying to turn red. Not always worth the color.
How high is high light ? Based on the info I cannot say that the light lacks a peak in the red spectrum.
Looking at your other posts I got the above. Don't know if it is up to date but it might help others provide a solutin.
1. In my opinion 2 T8 bulbs over a 125L (30g) is not high light, rather on the low side of things.
2. If accuraate, the NO3 are high. High levels of NO3 will prevent/help the plant stay green. But I do not recommend N stress to get red color.
3. Simple R. rotundifolia will get a pink-brown growing tip under high light. R. butterfly is way reder.
A red light will make the plants look redder. I cannot confirm at this time that red light actually makes the plants redder.