When changing water, make sure the newly mixed salt water is prepare overnight. Use a container and put a powerhead in it to circulate the water. The specific gavity, temperature should be within the same range as your tank.
To change water or not to change water, the debate would never stop. I never changed my water when I had the 90 gallon. Now with the 120 gallon tank I have, I do it once a month and follow the above procedure. I still add the necessary elements into my tank because my tank contains heavy fish and coral population. I find changing water an aid to my animals' health. However, if I do not change the water, I see no ill effect as well.
I would strongly recommend you get a big skimmer for your tank. FO tank actually needs the best areated water. Using tap water? I am not sure of the water quality in Taiwan so I can't say. But I personally prefer the RO water. Natural sea water is also fine if you know how to pre-treat the water.
I am also not sure what are those additives you are referring to. Semed like a lot of additives you are adding into the tank. Using a friend of mine's tank for example, a 300 gallon FO tank with large eel and Angels, all he is adding is Fish Soution from Ecosystem and Bionic from ESV. He has an Aqua-C skimmer, UV, and some live rock. He uses live sand as well. One of his Majestic angel fish is in its 8th year and still growing. And he changes water twice a month at about 15% each time.
HTH!