My ponds are tap water clear and most likely have no plankton food base for bluegill fry, and they all have aquatic weed problems (mainly chara and pondweed). But, what clear water pond doesn't have weeds,-
unless it's inside a cave or on the planet Mars ?
Fertilizing is like a two-edged sword with maybe the weeds the larger beneficiaries. I have access to liquid phosphorus (0-40-0), and, I'm wondering, should I forget about fertilizing or maybe, " Damn the torpedoes, full speed ahead" and just do it ?

Honestly, in your shoes I'd try feeding rather than fertilizing. Less risk that plants will go completely crazy.

But if you really want to fertilize, please have water checked for pH, alkalinity, and hardness first. If you have acidic pond with low alkalinity and hardness, fertilizing is futile. Lime it & get it in order, then consider fertilization.

Chara loves limey soils. Don't dismiss the productivity of chara ponds. Without fertilization they can be remarkably productive ponds particularly when combined with right kinds of forages. Chara is a host to the periphyton community and one member is a blue-green algae with which it is symbiotic. This algae fixes nitrogen and naturally fertilizes the Chara. Rice paddies do not lose their productivity, even over centuries of production, largely because of of chara species and their symbiotic blue-green algae. In addition to fertilizing chara, the algae fertilize the pond when recycled by organisms. An example of such organisms are PK shrimp which thrive in ponds with chara and with no additional fertilization. Chara tends to make water clearer, truth be told, it is the organisms living on the chara that do the bulk of that lifting.

Water clarity isn't always an indicator of poor fertility. You may have community of organisms that very efficiently graze and consume phytoplankton keeping the clarity high. We are learning that production in highly grazed systems are not adversely affected by the grazing. Under these conditions all organisms thrive.

Thanks for all the info on the chara plant - didn't realize that it had that much going for it. It apparently does have a filtering effect on the pond water. I tried spraying liquid Phosphorus on a small area of a pond where the wave action was minimal and which had previously been cleared of Pondweed with Aquathol K.(mainly as a test) . The ponds have a high alkalinity count (65 to 70 ppm) and the bloom appeared about 2.5 to 3 days later.