id really focus on your water source, as well as the type of salt mix being used. Some cheaper brands will not be as "chemically balanced" as others, resulting in higher readings in certain areas of the tank. Nitrate is the breakdown of everything added into the tank as well as natural occurances in the tank, fish waste, food, dead fish, etc. Without a doubt, your massive nitrate issues are coming from your water and or salt mix. Think about it, your doing 20% waterchanges once a week and the levels still are not going down. Even with stock in the tank, you should see a little change.
A few things id like to konw is the brand of salt, even if its a UK brand, amount of a fish in the 80gall tank.
What id also do is test your tap water before you mix it with salt, then test after mixed with salt and airated for 24 hours. This will determaine the amount of chemical change in that amount of time. all that is needed to be done is gather 1 gallon of water, test the water for you nitrate readings, copy readins, mix salt to desired SG levels (1.023-1.026) airate for 24 hours and test again. compare and then you see the difference.
In the aquarium Nitrate is removed by water changes and or DSB's Deep Sand Beds. Other ways such as the use of a refugium with live plants, mangroves, etc will reduce such levels.
