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Tank disaster

 I hope somebody can help me figure out what's going on with my tank and
what is to be done.

 Tank specs: -tank: 29 G, 80 l water volume, 15 l gravel volume (I use
liters for dosing the Dupla Fertilizers)
	     -substrate: 50 lbs "filter" gravel 1-3 mm size, bottom
1/3 mixed with 250 g Dupla laterite; Dupla heating cables 24V/25W with
DIY transformer
             -lighting: 2X20 W Ultra-Trilux in double-bulb strip light with
reflectorizing material, for a total of 2760 design lumens; 11 hrs/day
             -filtration: Eheim 2213 Plus filled with mechanical and
biological filtration material, output is spray bar 2" below water
level
             -plants: an assortment of fast growing species: several
varieties of Hygrophila and Echinodorus; Ludwigia, Bacopa,
Myriophyllum, Rotala and Lilaeopsis
             -algae crew: 2 2" SAE, 1 Ottocinclus, 1 2" Peckoltia, not fed
             -temp: 76 F
             -tap water: KH 4, GH 6, pH 7.6

 Here's the anatomy of the disaster: (I made several measurements
during each day, I will post only the ones taken at similar times of
day, all other parameters -temp, lighting- were kept constant)

Day -2: I treated the gravel with a 1:1 dilution of 31% muriatic acid
to eliminate any "lime chips" of which _very few_ showed up at the HCl
test. I know I shouldn't had, but I had a gallon of muriatic acid
sitting around and nothing to do with it :-). I washed the gravel
_thoroughly_ in small batches. It didn't smell like Cl at all after I
rinsed it if that makes any difference. It dried halfway in the next 2
days.

All the following events are approx 24 hrs apart, in the evening,
1-2 hrs before the lights went off

Day 1: I setup the tank, filled it with water, added 4 Duplaplant
tablets and 40 ml of Duplagan. In the evening I planted the tank and
later on I added the fish. I started injecting CO2 in the intake of the
Eheim from DIY yeast, 60 bubbles/min.

Day 2: Plants bubbling like crazy. Fish moderately passive but I
thought it was due to the new environment. Then
I noticed one big Malaysian snail totally imobile (never seen
something like that ever since I got them). So, I tested the water,
something I should had done _way_ before this incident.
       pH below 5.0, KH=0
       75% water change --> pH=6.6 KH=2.5, the snail (that saved my
fish) assumes his usual activities as I pour tap water into the tank.
Here's another use for trumpet snails, pH indicator :-)
I thought it was CO2 overdosing so at this point I stopped injecting
CO2. 
Day 3: Plants bubbling, fish and snails OK from now on. One drop of
Duplaplant 24/day added from now on.
       pH=6.4 KH=1.5
       90% water change (doesn't have chloramine)--> pH=7.0 KH=3.0
Day 4: pH=7.0 KH=2.5
Day 5: pH=7.0 KH=2.0
       frustrated, I added 1 tsp of baking soda -->pH=7.6 KH=3.5
Day 6: pH=7.3 KH=3.0 GH=3.5 (tap water again is pH 7.6, KH=4, GH=6)
In the remote possibility of dealing with biogenic decalcification
(but AFAIK it doesn't go that fast ,pH should rise not drop and it
shouldn't affect GH) I started injecting CO2 at a rate of 30 bubbles/min.
Tomorrow, if the KH keeps dropping I will stop the CO2 and remove the 
only piece of driftwood in the tank just in case.

Amazingly enough, the fish are very active, munching on invisible algae
and don't seem to be affected by this abuse. The plants are doing fine
but it might take a while until they show any signs of damage. For now
they're bubbling and growing. Lilaeopsis is spreading runners, the 
Echinodorus-es already put out a couple new leaves and the stem plants 
grew about 2" in this past week (I transplanted them with good size roots 
from my 10 G "jungle").

So, my pH, KH and GH are dropping. I *guess* the most logical
explanation would be that the gravel wasn't rinsed enough and is
leaching HCl. The daily drop in KH is smaller that in the first couple
of days so probably the tank is going towards equilibrium, but who
knows how long that is going to take ?
So, why is it happening and what should I do now ? Replace the gravel,
though this would be a torture; wait until the tank stabilizizes;
compensate the daily drop in KH with sodium bicarbonate to speed up
the buffering ?  And ,if I wait, does anybody know how long it might
take until the tank is stable ?

I will appreciate any suggestions or comments.

I apologize for this monster post, I just wanted to be thorough. I will
shut up now.

 Didi Soichin
 didi at wwnet_com
 Westland, MI, USA