Turning Yellow

I gather from all the discussion of "limiting factors" something must have
recently come into short supply in my aquarium, and I know I'm going to
have to experiment, but I was wondering where to start.  Something has
changed over the last couple of weeks.  Assuming these are related, this is
the order of events (1) the hornwort started to get a reddish brown tint at
the crowns (2) all of the new growth on one of the swords is yellowish, on
the other it's more of a pale (whitish) green (3) my foolproof Java Fern
has started to get yellow tips on its leaves.  The anacharis and especially
the moneywort (that's how it was sold, I assume from Rataj that it's some
sort of bacopa) are still growing rapidly.

I know the setup is not ideal . . . and I'm working on a new tank, but with
my budget these things take a lot of time.  Light is inadequate (around
1.4w/g).  There is a UGF.  The tank is a little overstocked with fish.  But
the plants were surviving with these before.  The only thing I can think of
that has changed is that I switched to RO water (to try to cut down on the
phosphates I was adding to the tank).  I'm having a bit of difficulty
bringing the KH up without bringing the pH too high, and this will only get
worse now that I have heard that the silicates in Acid Buffer are bad.
Anyway, I also have a yeast CO2 on the tank, but have had it for months--it
helped at first.  I add a little Flourish 2x/week.  Is it time for me to
buy some of the pricier test kits, like iron and phosphates?  Or should I
try something else first?