i have used natural salt water for all my brackish setups
only problem i had was with trying to grow mangroves in the tank :oops:
edit: just noticed you were in ashurst, a trip to the beach may cost more than buying the salt
weekly water changes will be heelping
a skimmer may help, but maybe expensive
putting fine airstones in may give an indication of if that would help
if the aeration created a scum or froth on the surface
doing all the right things mel
only leaves the silica sand really but that should be sweet
if you take a glass jar of water from the tank and let it sit for a day you may be able to see anything settle out of the water if that was the case
sorry no quick fix
maybe substrate leaching particles in water
bacterial bloom
uneaten food,
filter cleaned too well
just to mention some possibilities
questions would be what type of substrate, tank/filter maintenance, feeding etc?
so long as no stress bubbles are showing where front pane joins end panes you should be alright
if using a clamp may pay to put lengths of tinber and padding to spread the load front and back