If you dislike green spot algae, add lots of PO4.
If you like green spot, don't add PO4.

Lower lighting to 10 hours/day.
If things were doing well with 2.6w/gal, why did you change?
More is not better.

A small tank with lots growth needs tended a lot.
Adding more light will require more nutrients and will produce more
growth.

If you can angle or pull the light back away from the glas, that will
also help, unless you grow plants on the glass.

Get some KH2PO4.
www.gregwatson.com
1lb will last about 100 years for this tank.

Regards,
Tom Barr