With a __large fish load__ this does occur without water changes?
Try and you'll get BBA and Staghorn as well.

I can easily balance a non CO2 tanks and not do water changes either,
but not with a high fish load like Tony's tank and big old pleco.

Well, you can do it, but the tank will look like donkey....

With a balanced fish, this is not case here, it does work well.
But that is not the case here.

You are welcomed to add many large fish in the 20 gal and see how well
the plants do then.

The RO issue? No, the plants need GH/KH, those will be used and are
plant nutrients.

Plants export the waste, if you have too many fish for a tank, there's
a limit were the plants, no matter what you do, cannot keep up with the
fish waste production rates. Then you get algae.

That's why we cannot keep adding more and more fish to the higher light
CO2 enriched tanks to supply the Nitrogen needs.
We know it's not excess NO3, PO4, traces, if we add a little NH4 we
will get the bloom.
This is a repeatable test, try adding more and more shrimp to the tank.
You will hit a breaking point.
The closer you get to this point the more sensitive the system will be
to algae also(vs adding inorganic nutrients).

I suppose one person in the bathrom and peeing on the house plants
could work, but 10? Nope:-)

Regards,
Tom Barr