Weird kH/gH discrepancy...why aren't my plants growing? (long)

I've got a planted ~18 gal tank that I've been running for a year or so,
just put the CO2 back in it, ran it last summer but was away most of the
winter so didn't change the bottle then, only recently got back around to
it, but I'm wondering if my monstrously high gH is having adverse affects on
the plants and fish (certain kinds of fish flat out refuse to live in it,
like livebearers, however my bumblebee and neon gobies seem quite happy with
it, and there's virtually no salt).

Measured today:
kH: 3.5 (degrees)
pH: 6.6
gH: At least over 50, it's unmeasurable with my kit
Ammonia/Nitrite/Nitrate: nil/nil/50ppm (dosed the tank with PlantGro Iron
Enriched before measuring this, not sure if it has any effect)
Lighting: a bit over 1.5 watts/gal
CO2: 26.375 according to a web calculator

The water comes out of the tap about 7.6, after the first night with the CO2
on it dropped to 7.4, it's been running for two weeks now and the original
bottle's still pumping out loads of the stuff. Most of the plants don't
seem to be looking any better, though (other than my java moss that's
growing like crazy, as usual...anyone in the Toronto area want some?). They
grew like crazy last year, though. Substrate's 1/3 flourite, 2/3 gravel,
and I replaced the tubes yesterday (PowerGlo and SunGlo were originally in
it, replaced the SunGlo with a FloraGlo, and changed to a fresh PowerGlo).
I tend to leave bodies in the tank if they're not diseased and are
decomposing normally, assuming there's not too many at one time, mostly
because I can virtually never find them, the fish just disappear, and due to
a bad batch of cheapie rainbowfish, there's been a couple new ones in the
last week or two.

It's all pretty much the same as I was running it last year, only the CO2
diffusion seems a bit more efficient with the current reactor, the
concentration's higher than I usually maintained, and I'm not having algae
problems so I'm assuming my water's not loaded with phosphates...it comes
out of the tap hard as a rock, though.

So I'm wondering if it's something that's accumulated in the water that
might be causing problems, like minerals from the unusually high gH reading?
I really don't want to do massive water changes, too afraid of completely
throwing everything out of whack. If there's a chance the gH is part of the
problem, how could I lower that without phucking with my kH/adding
phosphates and causing an algae bloom/etc?

If it's not likely to be part of the problem, what else would be most likely
to be causing the problem? And should I somehow try and reduce the amount
of CO2 my tank's getting? It's a yeast reactor...

Thanks!