You have fish now so you will have to do an fish in cycle. Cycling a tank is very important and when doing a fish in cycle it is very important to monitor ammonia/nitrite/nitrate levels daily to keep levels from raising to levels that are harmful to fish. You need to do as many WC's as needed to keep ammonia and nitrites below .25. Here's a little article that might help...
I just learned about cycling but I already have fish. What now?! - Aquarium Advice
As for the DW as Fort stated you can take it out and boil it or you can add carbon or Purgen to your filter to help remove the tannins. 50% WC's often will also help with the tannins plus will help keep ammonia/nitrite levels low which is important for the fish.
One last thing is you should cut your photoperiod down to 6-8 hours daily so you don't have algae issues.