.25 ammonia isnt really to dangerous, but it could get worse. you should not have started with tangs. they are sensitive fish and should only go in a well established tank. ive never had one but this is always what ive been told. i would deffinitly take back the second tang and try to get credit or something from the lfs because your tank is going through a small cycle to make up for the new load. so you are going to have an ammonia spike but the question is how high will it go. i personally would take back both tangs and start with something hardier, smaller and less expensive, but if you want to try it with the 1st tang then take the second back and test your water regularly. those products you mention will reduce ammonia but only temporarily. you need to experience the ammonia spike to build up your biological filter. you could possibly try it with both tangs. you would have to test the water like every day and if the ammonia got high you would have to do a water change. this would give you a very long re-cycle but MIGHT allow you to save both fish and not have to take one back. however with tangs i would NOT recommend this. good luck and i hope you take back at least one of the tangs.
