Salinity should be 1.024-1.025 for a reef. You cannot keep chaeto in that hob filter unless you take the lid off and give it light somehow, it is a plant and needs light for photosynthesis. Otherwise you will just have rotting and decintegrating chaeto. Go to the store and get (or request) Chemipure. It is a bag of stuff to put in your filter. IMO you dont need to skim with such a small tank; water changes should suffice. Get a phosphate reducing agent to put in the filter with the bag of chemipure, and you could also get Purigen (you put like a spoonful in a mesh bag and also put that in the HOB).