You have a 2-bulb T5HO fixture with good reflectors on an 18" high tank... You are probably not low light.
In general, do not use watts/gallon guidelines on modern light fixtures. It has no correction for the higher efficiency of modern lighting.
If it is just sitting directly above your tank you are probably above PAR/PPFD of 50. (lower end of high light). If you have it raised well above the tank (20" above the tank rim or so) you might be in low-light territory.
See the graphs for the 4-bulb version of this lamp, having PAR/PPFD of about 115 at 20" height. You should be somewhere near half that (57.5-ish)
if your inhabitants don't like it, try reducing the dosage and watch for algae problems.
Fertilizers in general:
You've got good growth and no signs of deficiency, so you are probably OK for now. Keep an eye on the tank and see if things change later.
Your Flourish Comp has a decent amount of Iron in it. Not a ton but it is a decent amount, so you probably aren't severely low. The red ludwigia might get more red with a bit more iron, but you should probably figure out the amount of iron you're already adding.
Unless you're adding some elsewhere, you might need some. Flourish comp also contains *some* potassium, but compared to plant needs the amount is relatively trivial. Fish waste also adds some, but it too is relatively low in potassium. Watch for deficiency signs and add if needed.