Putting a filter from an established tank is not going to give you instant tank. even at a month old as you add fish you should do it slow. I might be reading your post wrong, but I think you have too many fish in too new a tank. Remember that you need to increase the bio load slowly. Your bacteria in the filter only accounts for less than 25 % of total. The rest is on the glass, ornaments, and mainly in the substrate. Using a seeded filter is the best help as you have done. I do it often too to help along the bacteria farm.
As far as salt goes, I add salt to all of my tanks (1 teaspoon per 10 gallon water) except those with tetras. If I suspect illness I increase to 1 tablespoon per 5 gallons temporarily with an increase in temp. Seems to work for me.