Possible substrate problem

I need a little advice on a minor problem I'm having with my pH.

Before I set up my planted tank about 2 months ago, I read 
everything I could on the web (including the Krib) and in the couple 
of books I bought. When I setup my tank, I used 40% de-chloriminated 
tapwater (pH of 10 immediately out of the tap, 8.5 after an hour and
de-chloramined), and have been doing 15% RO water changes regularly 
since then, hoping to slowly bring the post-cycling pH down from 7.5
to just below 7.0.  While I was able to bring it down to 7.2 after 
changes, it's still bouncing up to 7.5 or above in about a week.  

My fish and plants don't seem to mind being at 7.5 (they're used to 
it by now), I think my plants can do better below pH 7 and I will be
able to add a mate for my Krib and a pair of Rams without fear of 
them dying off (already two female Kribs have not survived a week in 
my tank). It's easy to keep hard-water cichlids in Austin tapwater, 
but I'd like to evolve my tank into a community river tank.

I'm trying to lower the pH of the water with a nylon hose full of 
peat moss in my power filter, which only works temporarily.  I 
believe what's causing my pH increase is the blasting gravel I bought
when I set-up my tank.  I have two bags (20 lbs?) of the gravel in my
45 gal. breeder tank, along with 15 pounds of the fine, coated 
aquarium stuff.  I washed everything clean when I started, but I may 
not have done a good enough job on the blasting gravel, even though I
tested it with vinegar to make sure it wasn't calcacerous.  

I could tear down the tank and replace the substrate, but is there a 
way to avoid the agony?  Will adding more peat to the tank itself 
help?  Like I said before, the tank's doing great right now (I've 
even fought off an algae bloom when I added too much Ferro-Vit one 
week).  My concern is that I won't be able to introduce new fish and 
more difficult plants. I was trying to lower the pH with CO2, but at 
the current KH, there's no way to get to pH 6.9 without making 
Fish-Cola...and the KH has been stable through all the water changes 
(GH has dropped from 10 to 7).

Tank essentials:
45 gal. breeder, 1 1/2" mixed gravel subst., 2 clay pot halves
boiled driftwood log, sm. slate slabs
Aquaclear 300 w/sponge filter (running on low: ~150gph)
over 100Watts light, yeast DIY CO2, Substrate Gold laterite balls
lots of assorted plants including water sprite, duckweed, Anubias
6 Danios, 3 SAEs and 1 lonely Krib
pH 7.5, KH 6, GH 7, NO3 0 (going to start PMDD when pH is fixed)
No chemicals added except Ferro-Vit and Amquel (to initial tapwater)