Hajra,
Post your tank info in this thread...
Im going to try to be gentle here cause i am not sure how you willtake this advice..
Im thinking you really have to lose the castle.. sorry.. it's got to go..
couple of issues involved.. didnt want to say stuff before cause i didnt want to jinx or get you all paranoid...
but, that type of stuff isnt made for saltwater aquariums.. thre could be issues with paints, and chemicals inthe porcelin... and the objects need constant cleaning and then you are putting stuff into the water column
the purpose of the live rock is for many reasons, but cleaning isnt necessary... any food or algae or detritus that gets on it is digested from withint and from the fish and critters...
all the castle does is collect and store nutrients. (nutrients arent good)
also, not sure what gravel you have onthe bottom, or sand? but it needs to be aragonite sand... it doesnt have to be live, it will become live but that takes time...
your tank is cycling, constantly.. lilke a pendulum that swings back and forth. and only when it is in the middle is it good, which is only 30% of the time.. usually pendulum slow down, (for different reasons.. a real one slows due to friction and gravity... a chemical one can be more wild and more 'free swinging' and can swing for long periods of time...
ammonia is why your fish are gulping.. they are breathing poison.. just like if you were to breathe clorine... same affect.. burning, and no ability to saturate o2
I would remove the castles immediatly.
i would do a 75% water change, asap..
If yo udont have aragonite sand, then get that other stuff out.. that is probably why your pH is so low... aragonite is a natural pH buffer..
btw: aragonite sand is not silica sand/playground sand...
you need more live rock...they have some nice stuff, costs more but nice looking and can give you a lace/open look, which i think you might like, (kinda like the castle in that there is plenty of swimming in and out stuff)???
Anyways, im hoping you are comfortable with the suggestions so far..
I would also like to get more info on your filter setup... again, i fyou post in this thread i might review it easier