If you have a settled Ph of 5.5 you must have an almost non existant Kh
level - very close to a crash I would have thought - either that or you
are
adding CO2 (or still have CO2 in the water from the peat filtering). I'd
be
interested to know what you'd get if you take a bucket of your Peat
filtered
water and run an airstone in it for an hour....

I am still awaiting my new KH test kit (difficult to get hold of around here
for some reason). All of my tanks are currently filtered by an air driven
sponge filter of some sort, so there is plenty of aeration. What I will do
is to record a sample of water for pH and hardness before adding to the tank
and I will then record the same when I do a water change 24 hours later and
we can do a comparison.

Mark