Question
Hello Robyn,

We have a fantail goldfish that has been living in a 5 gal. aquarium since May. When I bought him, he was maybe an inch long - not counting the tail. Today he is about 2 to 2-1/4 inches long and still growing rapidly.  He obviously needs bigger quarters.

We  want to put him in our pond which is approx. 275 gals and now has 7 common goldfish in it which are about 3 inches long each.

What would be the best way to introduce him to the pond? I thought of putting some aquarium water in a container then slowly adding small amounts of pond water to acclimate him. How long should he go through the acclimation process before being put in the pond? I worry about ammonia buildup if he is in a small container too long. What size container would you recommend using? (I do not want to add any pond water to the aquarium as we want to get another fish and the aquarium is cycling nicely.)
I test both pond and aquarium water weekly for ammonia, pH, nitrites, nitrates, hardness, etc. and would make sure the water temperature was the same for him.

We抎 appreciate your input on making his move a safe one.  

Answer
Your idea of adding pond water to a container with the fantail and some tank water is a good way to get him used to the pond water chemistry and temperature.  If you use the same water to fill up your aquariums as your tanks, then this process shouldn't need to take more than a few hours.  The ammonia shouldn't build up that quickly if he's in say a 3 gallon bucket half full.  Cover it so he can't jump out.  If you're worried about ammonia, you can add some Ammo-Lock.  I've moved fish from inside to out by bringing them outside in a bucket, pouring the water and fish into a net, and then dumping the fish right into the pond so I'm guilty of not acclimating them but have never lost a fish from that. So, I advise one thing and do another!