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Substrate problems

Hi folks

I have been quite contented to read the postings on APD and have learnt
quite a fair bit (keep up the great information exchange). Unfortunately, 
I've ran into a potential problem with my substrate...or rather my gravel, 
as I've started my 30G tank before I started articles & postings on planted 
tanks. 

My tank is setup with 1/2" of Sera's Floradepot (for 1/2 the tank), while 
the other 1/2 of the tank has a network of PVC tubes with RUGF filtration. 
Then the entire bottom is covered with 3 - 5 mm size of river gravel, about 
3 - 4 inches of it. Further filtration is provided by an Eheim 2228 canister 
filter (a bit oversized)...but it works. CO2 fertilization with DIY yeast 
fermination, and weekly fertilization with Tropica's master grow, and some 
laterite balls placed in substrate. So far, there was not been any serious 
out-break of any algae, and the tanks's been setup for about 2 months now.

The tank is heavily planted with :

- Anubias Barteri var. nana (about 10 rhizomes)
- Anubias Barteri var. barteri (4 rhizomes)
- Hygrophila salicifolia (6 stems - doing well)
- Hygrophila polysperma (5 stems - not doing so well, new leaves are small)
- Hygrophila difformis (6 stems) 
- Bacopa caroliniana (4 stems)
- Echinodorus amazonicus (5 plants - medium)
- Echinodorus barthii (3 plants - medium, with one flowering)
- Echinodorus horemanii (1 medium plant)
- Cardamine lyrata (4 stems)
- Rotala macrandra (used to be 5 stems, but now only 1)
- Rotala wallichii (8 stems, but with some hair algae)
- Vallisneria torta (about 10 plants)
- Glossostigma elatinoides (about 30 - 40 plants, 1 to 2 " tall)

The fish load is also on the high side, with :

- 10 neon tetras
- 2 albino tetras
- 4 runny nose tetras
- 6 black neon tetras
- 2 small rainbow fishes
- 3 ottocinclus (albino)

But the good new is that I have no measurable ammonia, nitrate reading, 
nitrite is < 0.1 mg/l, and the water temp is around 30C (86F), and cooled
in the afternoons by addition of ice blocks. The problem and bad news is 
I can't seem to get my PH down....and it hovers between 7.0 - 7.5 PH, 
and I would like to get it down to about 6.5 if possible. I then bought 
a KH test kit and started keeping tab on this, and it hovers between 
7 dKH after a 30% water change to 17dKH inbetween twice monthly water 
changes. This is without any buffering and I know that my tap supply is
soft. 

I then suspected the gravel, and tested it with some vinegar. Wow, immediately
after addition of vinegar, about 30% of the the gravel started
(mostly whitish looking pieces amongst the brown pieces) bubbling...and 
probably the cause of my KH and PH going upwards.

Now the issue is should I tear down the tank and discard the gravel, or 
should I just continue since it appears to be a natural buffer for KH..and 
if I perform weekly water changes, I would be able to keep the KH to about 
7 or 8 dKH. And, how can I ever get the Ph down to a preferred 6.5, since 
IMO the gravel would almost certainly cause it to go upwards?. Also, should 
I start testing for general hardness as well....?

Sorry for the rather long post, but I just needed to get this out of my 
chest.

TIA, 
                                               
                            ^^^^^ 
                           ( o o ) THIS IS NOT ME.....   
<------------------------o0O-(_)-O0o-------------------------->
                                            
     Name  : Roy Cheok
     Tel   : (65)-484-0201
     Email : roycheok at cyberway_com.sg