tank water problems - please help

I have a tank 5' x 2' x 2' which has been set up for about 15 yrs and
contains community fish and is illuminated by 3 x 40W triton tubes (2
tubes on for 10hrs/day and the 3rd comes on for 3 hrs/day in the
middle of the 10hr cycle). The tubes are less than 1yr old.

I used to replace about 1/3 of the water and clean the filter every
couple of months and all seemed fine.

For about the last 14yrs everything has been fine then about a year
ago I started getting a dark blue/green algae like fur (about 1" long)
growing in clumps on the glass and covering the plant leaves.

I increased the frquency of the water changes gradually (assuming 'old
tank syndrome') and I am currently changing about 1/3 of the water
every week, but still the algae grows and the tank generally looks
under the weather.
For the last few weeks I have not used the 3rd triton tube.

I have undergravel filters fitted with small powerheads but after 15
yrs, they don't work efficiently any more.

2 Airstones pumping a lot of air.

Eheim 2217 Classic (1000 litres/hr) external filter which has coarse
substrate at the bottom, coarse filter pad, smooth substrate, filter
wool and finally a bag of carbon on top. I now clean the substrate in
tank water every 2/3 weeks and replace the filter wool and carbon at
the same time.

The tank was well planted, so much so that I had to remove handfulls
of plants every couple of weeks to keep it under control.
Excessive plant growth no longer occurs, probably because the algae
covers most of the leaves.

Many plants died and I restocked just before Christmas, but many of
the new plants have subsequently died.

I live in a hard/very hard water area.
Water testing has shown the following-

on 3 May
Water temp - 79F
pH - 6.4
KH=TAC=Carbonate hardness - 3 degrees
GH=DT=TH - 21 degrees
NO2 mg/l - 10
NO3 mg/l - 500

After this test I changed 1/3 of the water and cleaned the substrate
in the filter and replaced the carbon.

on 15 May
Water temp - 79F
pH - 6.4
KH=TAC=Carbonate hardness - 3 degrees
GH=DT=TH - 21 degrees
NO2 mg/l - 1
NO3 mg/l - 500

Any help to get my tank back to good health will be much appreciated.

Many thanks
Pete