i should do the replacement every 2 weeks right??
Bluesi4, I don't want you to think that I'm coming down on you because of the setup you have and the methods that you are using. I was once in your position before and would like to provide you with the information that is necessary to setup and maintain a thriving Marine Aquarium.
First thing is first, you need to upgrade some of your equipment in order to provide the correct environment. I'm not sure of your financial situation but as mentioned before this hobby can be very draining on your wallet and purchasing the right equipment is definitely going to save you money in the long run. Don't waste your money on cheap items because you will just end up replacing them in the near future and spending more overall.
Water is the environment that your fish live in and is quite honestly the most important part of your Marine Aquarium. You are definitely going to want to purchase and RO/DI system to hook up to your faucet to purify that tap water. Conditioners are sold that you can add to tap water but my personal opinion is that they do not work. Even if they can nulify some chemicals they still cannot remove sediments or harsh metals that remain in the water. An RO/DI unit will give you beautiful blue water that will be much appreciated by your aquatic inhabitants. Also the tap water contains ridiculously high rates of Phosphates which will just add to Nuisance Algea thriving in your Marine Aquarium such as Cyno Bacteria (Red Slime Algae) or Green Hair Algae. Both do not look anywhere as appealing as your may think.
Your Substrate is where you may want to start next. Crushed coral is definitely cheaper then Aragonite Reef Sand but it allows unnecessary amounts of detritus (waste) to collect in the nooks and crannies and will sooner then later effect your water quality and buffering capacity by increasing your Nitrates, Nitrates, Ammonia Levels and the3 ability to maintain a stable pH level. I would suggest purchasing a bag of sand and slowly replacing portions of the Crushed coral with the Reef Sand.
As far as the marks on your tang go. It sounds like he was definitely under nourished in the store you purchased him from. I notied you stated Pet Goods in your post. I personally do not buy from them because of the lack of care that is given to their Livestock. The tang is suffering from Hole in the Head due to lack of nutrition. It will eventually clear up by providing a steady diet of Nori and it wouldn't hurt to help soak it in garlic for a few before introducing it at feeding to help build the Tangs Immune System since it appears to be just a baby.
Any other help just drop me a line.