When you use Excel, you are not doing the non CO2 method even though technically it's not adding CO2 gas, it still breaks down into CO2.

You can do and should do large weekly or once a month/2week water changes, you need to dose more and a source as Vaughn said, of PO4.

I'd go 2 weeks and do 50-60%, dose about 1/4 EI(2x a week or 4x every 2 weeks).
That with a decent fish load ought to do it.

When you add Excel, it increases the growth about 3x that of a non CO2 tank, so you need to add more nutrients. That means a higher change of build up, so the water changes come back into the process.


Regards,
Tom Barr