Hell, I throw away Blyxa by the bucket full.
I have it in 2 different tanks and it does very different growth patterns.
One tank has this massive greener growth, I take out a full 5 gal bucket worth monthly. Gets tall, bushy etc, the other tank get dense and stays shorter, redder. Dosing is the same, duration of light is the same, PAR is the same, sediment is the same and same age(ADA AS).
2 main differences, one tank uses older coralife 6700K, CSL 8800K bulbs(no longer made unfortunately) and the other tank has giesemann midday T5's. the other different is the current comes smacking down on the redder short blyxa along with CO2, so the higher current seems to cause more change in morphology.
I like the slower red growth, the other tank is too weedy.
So I am increasing the Staurogyne instead.
Most other plants do fine in the other tanks, no real differences to speak of, some color changes etc.
While current is not ruled out, the color temps of the bulbs might be some of this.
With the GE 9235K bulbs, some reds pop out really nicely.
With the gieseman, a similar thing occurs.
PAR is the same, growth is, or even better than the nicer color bulbs that produce a nicer aesthetic.
So I might be getting better light use efficiency from one tank, but better coloration, growth patterns in another, but not with other plant species.
So some of this seems plant specific.
I've had Bylxa get really bushy thick with plain sands also, flourite etc also.
Your Blyxa seems like you could use some thicker bushy growth. I was not sure if you had just replanted and it would grow in thick in a month or so, or it was normally sort of thin.
CO2 is part of it, current is some, but once the plant gets going, it should do quite well. Mostly once the roots get established, that's when it does well, so good stable CO2 seems required.
I've had it get scraggy like the tank pic above however, but it was a CO2 issue, or the fish uprooted it often.
Increasing the CO2 and current on that spot took care of most issues.