Strange water values, hypothesis

Hello! This is me delurking. I have some strange and nonoptimal water
values, and before I do things to correct them, I want to run my
hypothesis by y'all, as my proposed correction might have bad effects
if I'm wrong. (interest piqued yet?)

First, my tank parameters. I have a 50G (36" long, 14" wide) tank. The
substrate is roughly 2.5mm gravel at a mean depth of maybe 3-4". The
lighting is four normal-output 30W Triton fluorescent bulbs (CoraLife
fixture, very nice, btw). They are lit for approximately ten hours
total, with a break at midday of about an hour. The tank residents are
eleven Rasbora heteromorpha (Harlequin fish at roughly 1.5"), and two
Trichogaster trichopterus (three-spot or blue gouramis at roughly
3"). The tank is roughly six months old in its current location,
around two years old total. The plant residents are roughly 10-12
Hygrophila polysperma, and a few specimens of another species I can't
identify positively, but which I think is Egeria densa.

The normal water values are: temp 76-80F, pH 7.0-7.2 (LaMotte),
carbonate hardness 1.7dKH (LaMotte), nitrite 0 (Tetra), nitrate 12ppm
(Kordon, both their old plastic pillow and their new foil pillow solid
reagent), phosphate 0 (Tetra). I have had occasional blooms in the
past of cyanobacteria or green film algae. I also have a little spot
algae, which I consider normal. Water changes are 50% every two-three
weeks, about which I'm pretty religious. I do not inject CO2; the
water is very lightly aerated. I don't fertilize, and there is no
laterite or other iron source in the tank. Can you say, "fish owner
late come to plants"?

Lately the temperatures around here (Seattle) have been high, and the
tank temperature has been straying high during the day as a result.
Sometimes it gets up to 84-85F. My water chemistry has been changing
rather drastically too. In the past three-four weeks the following
values have changed: temperature (as noted). pH has risen to 7.6.
Phosphate has risen to measurable levels (.1ppm). And, here's the
weird part, nitrate has bottomed out at zero. I can't get a nitrate
reading on any of the three test kits I've tried.

Here's my interpretation. In case you haven't guessed, the Hygrophila
are growing quickly. I am also removing a large amount of green film
algae from the tank sides and from driftwood. I am guessing that the
plants (the algae and the vascular plants) are sucking the nitrogen
out of the water faster than the fish are putting it in. (I feed
sparingly). They are also blocked on CO2 roughly as much as nitrogen,
so are removing nearly all of it from the tank, which is causing the
pH to remain high. I have no idea why the phosphate value is high.

My plan of attack for the short term is to reduce the lighting for a
while, aerate the water more (to get CO2 _into_ the water, of all
things) and, God forbid, feed more heavily.

For the long term, I plan to inject CO2, since apparently this is a
blocking factor. I will be buying CO2, iron, and perhaps potassium
test kits soon, with an eye towards setting up a "high-tech"
(cannister) CO2 injection system. I have been planning and putting
this off for a long time, nearly six months. I will probably have to
add sodium bicarbonate to keep my carbonate hardness up and my pH
stable, big deal.

Does this sound reasonable? Or am I daft? The reason I'm worried about
my water is that I know with these water values I am ripe for
cyanobacteria to slime every surface on the tank. The last time this
happened, I had a dieback of the Hygrophila. So I'd like to avoid this :)

Thanks for any ideas,