hello everyone,
i have a puzzle.

in my 15 gallon aquarium, which is about 3 years old, i have;
breeding pair ancistrus (5" long)and their offspring usually twenty
ish tiny ones which go to the lfs at about 1" long,
2 siamese alae eaters / siamese flying foxes (4" long),
2 upside down catfish (2" long),
7 harlequins (1" long),
2 neon tetras (1" long),
1 whiptail catfish (4" long)
1 pygmy puffer (1" long)
1 large amazon sword plant and an abundance of reedy plants which i
occasionally pull out to keep them under control.


I used to have 3 pygmy puffers, but about six months ago 2 of them
died in the night. I checked the water and found the ph at 6.2 and the
kh at 4 degrees.
I did a partial water change only to find that the ph and kh dropped
agian over the next few days.
Since then i have been doing 40% water changes every week to stop the
ph dropping to a dangerous level. I have done a fair bit of reading
about ph and kh, and i understand ( from a fish keepers point of view)
the relationship between them.
At about the same time as the ph kh dropping started i noticed snails
in the gravel, their numbers grew rapidly to a few hundred ( i guess)
and are now stable.

The only process i know of that will cause kh to drop and then the ph
to drop is the production of an acid, and i think the acid must be
nitric acid produced from the nitrates present in the water.
The puzzle is this.
Are the snails causing too much bio load on the system?
Am i over stocked?
Am i over feeding?
or is there another possible cause for this drop?

I feed once a day an hour or two before lights out; 1 very small pinch
of flake food, one eighth of a lump of frozen glassworm (really only
for the puffer, but they all eat it)one algae wafer and one carnivore
wafer (the wafers are sometimes substituted with courgette (zuchini)

Sorry to ramble on but i see so many posts where the information
needed is not provided.

tank parametes after a weekly waterchange: Ph 7. Kh 9. nitrates 90ppm.
nitrites 0.

just before a water change the nitrates and nitrites remain the same
but the kh has fallen to 4 and the ph is 6.2 - 6.8.

any help gratefully received

chris nuttall