What stopped them?

I have a 15g with 20 W daylight and 36W warm white over it. I have a gravel
substrate, UGF and a heavyish bioload. There are sword plants, vallisineria,
Elodea (densa? crispa?), rotala? (red on top) and eel grass? (sorry about not
being specific, the shop has no idea about plant names, although it stocks a
reasonable range) All these plants grew quite adequately (especially the Vallis)
for a couple of months and then stopped. Then light green hairy algae took over
the tank, not attached to anything in particular. I thought that I would let
this grow to take out as much of whatever there was an excess of fronm the tank.
The higher plants as I say have stopped growing, and they are now a lighter
green than they should be except the eel grass which, where it is still alive?
is nearly black. I have started putting in "aquarian" brand plant food. Could
anyone tell me what has stopped the plants thriving? I would guess they have
used up most of a certain trace/nutrient but which one? My tap water has high
nitrates, 40ppm, so I do not do water changes  very often.

My father once said to me:
"Son, there comes a time in every man's life when he has to make a stand."
I have just bought a new fish tank, so I guess my time has come.
I'm off to the shed.                            (Apologies to Stephen Fry)
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