Little problem

hi all and thanks in advance for trying to help
ok here gose
i have a 4ft,2ft,2.5 foot tank
this tank also has sump
i have around 130 kg of live rock and i have had it for a year now
i have 8 fish/inverts
blue tang
brown tang
2x clown fish
rock goby
1 angel
1 puffer
30 hermits
and another fish that was given
anyway
i have a ocan runner 2500 power head
turboflotor skimmer with ocan runner 2500 running it
laguna 4000 to return water to my tank
and a fluvel 205 external pump aswell
ok now you know what i have this is my problem my tank has been running now for around a year and i have never had a problem at all
this last 4 week im getting amonia at 0.5 and i have tryed 20% water changes all the way up to 50% water changes
i have tryed changing there food
water from different sauces
changing skimmers
changing power heads
i have changed everything i can think of
even changed the pipe work too
my water test have been done with many different test kits and all show the same
i have done every test ph and so on and all are ok
so can anyone help me becouse this one has me
i have no idea where its coming from
thanks lee

 

was the sandbed stirred in any way?

 

Do you have detritus collecting anywhere in your sump/fluval?

 

due to me messing around these last 4 week sandbed has been move
i have about a 1mm lair of detritus in my sump
sump is 3ft,1ft,1ft
i have just today put a power head in the sump to throw the detritus up for the skimmer to take out

 

and the amoina never seems to change or covert