75 gal. freshwater community phosphate levels high

Phosphate levels off the charts, pH 7.2, ammonia 0, nitrite 0. 75g
community tank with cichlids, barbs, tetras, various plants. Green
algae bloom, green stuff on substrate - is it bacteria? Algae hair
growth. This tank has been setup for one year.

Fish fed once per day - maybe a little too much food from sources on
internet, books.

My main concern is phosphate levels. Tap water used for changes has low
phosphate readings. Tested levels in tank are off the charts and have
been since testing began 12 weeks ago.

Treatments:

1. Water changed 4 times in previous weeks, anywhere from 20% - 50%, no
effect on phosphate levels.

2. Water used to change treated with chlorine remover. Phosgard and
phos-x used for 6 weeks, no effect whatsoever on phosphate levels.

Phosphate testing rig used to test tap water and filtered water from
refrigerator, results consistent.

Those are the facts. Here are the questions:

Is the phosphate level of grave concern?

Treatments listed above have been ineffective. What other treatment
methods are available?