Hi booboo,
When you say "dusting" does it resemble as though someone sprinkled salt over the fish?
If this is the case it's a sure sign of ich (or white spot disease)
Best thing to do is go get some medication designed to treat this. My personal favorite is Maracide by Mardel. Don't know if that is available where your at.
Another treatment some use is 1 tablespoon of salt per every 5 gallons of tank water. Make sure it is non-iodized salt. Table salt will work or you could use kosher salt.
If using this method, take some tank water out of the tank and mixx the salt throughly in this water and pour it over the entire length of the tank.
Also make sure you allow for the water displacement in your tank. This means any gravel, decorations and such will displace water.
Say you have a 30 gallon tank and you have gravel, some rocks or other things in it your actual water content is probably around 24 gallons or so.
You will also want to raise the water temp up to about 85 degrees. Do this gradually.
Ich is only treatable when it has left the fish to multiply. By raising the temp this will speed up the process of the disease leaving the fish.
What ever plan you choose continue treatment for at least 5 days.
I just read your other post about adding the angel fish. Did your SD have this "dusting" before or after you added them? If it came down with it after, they (the angels) probably had it and gave it to the SD.
Check your angels very carefully. You should always quarrentine new fish for at least 2 weeks in a seperate tank to watch for any signs of disease berfore placing them in you established tank.
Keep us posted on how it's going...
HTH