Hi
Not sure why you think plants don't use 30ppm. The plants we buy are mostly grown emersed, in atmospheric CO2 which is pushing 400ppm, we put them under water and they struggle to adapt for a while, in 1-3ppm with no CO2 supplement . 30ppm is as much as we can use without harming fish, it is a compromise, not abundance!
Does fluctuating CO2 cause algae? Lots of threads on here about it, some don't think it does.
But my experience is that BBA is caused by fluctuating ( ie going from high to low) CO2.
I have 6 tanks, various algae at various time, but BBA in only 2, and these are the only 2 that I have used CO2 in.
One was. 50l with DIY , BBA started from almost day 1. Stopped the co2 months ago but BBA still going strong.
The other is my 350l tank. I tried DIY in this, had no effect, no BBA probably because I never managed to get levels very high. But when I changed to compressed co2 it was fine until the reactor jammed, effectively stopped injecting for a couple of days and BBA was triggered.
And the trouble with BBA is that once triggered it does not go away even if you correct everything.
Why does fluctuating CO2 cause algae? Hopefully Ceg will give you a proper answer, but my guess is that algae are caused by lots of things, mainly light, but also promoted by waste products. When plants are adapted to a particular light level and co2 availability, and then the co2 level suddenly drops they struggle and leach waste products, that triggers the BBA.
I would advise against DIY co2. Compressed co2 is really quite easy to set up.