Thusly (chris nuttall) Spake Unto All:

(which should have the opposite effect i think). The ph and kh only
start at those values, they soon fall to much lower levels.

Do you trust your kits, though? The reaction chemicals have a shelf
life of perhaps a year.

i have tried bicardonate of soda, the ph and kh still fall, they just
start from a higher value, i tried evian too, with the same result.

Evian is essentially bicarbonate dissolved in water with some salt
added, so it's the same thing. Adding bicarbonate in any form is a
temporary fix, as the tank will eventually equilibrate with the
atmosphere by releasing CO2, and the KH drops again.

I don't remember what pH/KH you want, but if pH 7.5 is acceptable to
you, just plonk in some limestone. Crushed coral, seashells, even
cuttlefish bone or eggshell will work too, and you can if you want put
them in a bag in your filter. But the easiest is just to decorate the
tank with limestone. Limestone will raise KH and pH and lock pH at
7.5.

i think i should get some coral, then as it dissolves there will be
more 'in reserve'

That will work yes.

chris