I have not found Proper PH to be effective.
What are your filters? Do you regularly clean them or rinse them with
de-chlorinated water?
Do you do regular water changes every week? As a new tank owner it is
important to do change more often
What is the pH of your tap, well or whatever water you use to top off
or change water with?

What is your substrate, gravel?
What is your kH? It should be a minimum of 80 ppm (160 for mollies is
better).
I recommend Wonder Shells for kH, as they add needed electrolytes
besides the necessary calcium. But it seems there is something else
going on here considering your tanks age.
Make sure you are not adding low pH water to your aquarium, discontinue
use of the Proper PH and do a few 20% water changes with water of a pH
of 7.5

Hey carl thanks for help.

1st my filter is just the one that came with the tank. (has a small bag
of what looks like gravel in it). Ive just cleaned it for the 1st time
in the water that i had taken out of the tank 2 days ago.
As my tank has only been running a short time ive probably been doing 2
- 3 water changes a week. with tap water (Ph 7.0) with declorinator
init. More in last week (at least 1 daily) since ph drop.

I have aquarium sand in the bottom as its meant to be good for the
plants to root in. I have no idea what my KH is but i have been getting
my water tested almost daily at Peter Barratts the garden centre where
i got the tank + fish from. They have said that my salt is high and the
ph low but other than that ok.
However they did give me some white chalk type rock to put in (for
free) as it'll help to balance my ph out. (apparently.)
Last time they checked water they said salt had dropped abit, But ph
was still low.

Ive just checked water and ph seems to have lifted to about 6.4 since
adding the stones.
Will this balance the water out for good or is it just a short term
fix?
Is there anything that i can do other than frequent water changes??
Since my last post ive lost few more mollie young (now only 7) would
appreciate all help thanks again for your time.

Ross