your asking which to add first, dechlor or tap water conditioner??
cos they're generally the same thing! there are a lot of different brands some of which do various other things too, but the basic purpose of tap water conditioner is to dechlorinate the water!
Agreed, add the dechlorinator to the tapwater, very, very important,
before you add the fish.
In addition, 6g is qute a big might small for a single goldfish. I know, I know they are advertised in tiny little bowls, but that type of "care" isn't so hot. Goldfish are cyprinids, a relative of the carp. Some cyprinids are quite messy and produce copious amounts of waste. They tend to prefer cooler, heavily oxygenated tanks, so your double pumps are a wise choice. There are big cyprinids, like the goldfish and there are smaller cyprinids, like chery barbs. They display a lot of the same behaviour, but a cherry barb is a fish that is much better suited to a 6g aquarium than a golffish that even as a fantail, can be over 8 inches long. I kept goldfish for about 5 years and I kept two, only two, in a 45g, and I was planning to upgrade them to a 120g before a hurricane killed them. Literature and opinion vary, but the minimum for 1 fancy golfish is between 20-30g, and then you add an aditional 10-20g for each additional goldfish. So, for your single fantail, a 20g would be bare minimum, and I would add a lot of filtration, water circulation, and be up on water changes.
Goldfish are wonderful fish with great personalities. If you want to keep yours, consider upgrading his/her tank, or give him/her back to the LFS and opt for a smaller barb species, like a trio of cherry barbs would be great once the tank is cycled. Or, if you feel you're still new to this hobby, a single betta would love that enviroment (nix the second pump, they don't like that much current) and if you were up on your water changes.
I hope this helps a little bit. Welcome to TFF.
llj
