There was no phosphate in the water at all.

Well, there's your problem.

Gee, for years other people told me that the cause of BBA was too much
phosphate. Now you say having no phosphate causes it. This is typical of
the conflicting information I often here about BBA problems.

Obviously you are of the camp that believes you can control BBA with the
water. I bought into that mentality for years, and fought the BBA using
that method. The BBA won. Maybe for some people, their water, their
plants, and their type of BBA, that method can have some success. But it is
very clear from everything else I have read that more and more 'water
treatment' people are becoming convinced that BBA is a different kind of
problem entirely.

I have seen really nice show aquariums with tons of plant growth and no BBA.
When I ask the aqua person how they did it, they tell me: "I rinse every
plant in Chlorine before I put it in the tank - I don't let BBA in the
tank". That is how they handle it, they prevent it.

Once it gets in it's like cancer. Some may go into remission, some may go
away, some may respond to therapy, but in most cases it will keep growing.

I've had plant growth through the top of the tank. Swords with 24 inch
leaves, and so much plant growth I had to prune plants every week. Yet the
BBA never went away. I've read articles from other people who believed that
water control could combat BBA as well, yet they were left scratching their
heads when they lost the fight and the BBA kept growing.

I do not know if you have any BBA in your tank(s) right now, but if you
would really like to test your theories in practice, I would be happy to
send you some of mine. I can send you a rock or plant with the BBA species
I have. Just let me know.

Regards, Chris.