Thanks for the replies, I have replaced the sponge at 3 month points but I have to say I've not rinsed them each time I've done water change as I thought that would dispose of good filter bacteria. I have added some additional seeded nuggets to the first stage basket to try and increase filter capacity.
Feeding - I'm giving a small pinch of flake in the morning and half a cube of frozen mysis in the evening which i rinse and dry before feeding.
Test kits - tried API but struggled with colour difference so using Salifert.
Coral food is Salifert liquid.
No sump just the Hydra Filtron 1000 canister filter.
I will go for the larger volume water changes to try and get it down steadily.
you don't need replacing the sponges just rinse them with a couple of litres of tank water in a bucket you won't lose any of the good bacteria.your frozen cube just rinse in ro water and then drain it back in a small strainer and then feed . it has been known frozen food have nitrate and phosphate levels.
if above is followed with reg water changes your levels will get lower.