Question
I recently purchased a 90 gallon tank that I would like to use for the koi I have had in an outside pond for three years.  I am selling the house with the pond, but do not want to leave the fish behind.  What is the best way to go about this, keeping in mind I have several summer months to move these fish.  (7 fish in total)  The pond water was established with well water, the new house is on city water ... which I'm sure presents more issues to deal with.
Thanks for any suggestions.

Answer
You want to keep the 7 koi in the 90 gallon indefinitely or just until you can make a new pond?  How big are the koi?  Did you want advise about maintaining the new aquarium or just moving the fish around?  Do you have access to your new home already?  If you do, then set up the new tank there.  Use a container larger than the koi to move them to your new house.  Cover them in water over their backs, and add a battery air stone if you can.  Cover the container to reduce jumping and splashing.  Bring buckets of the old pond water if possible.  Fill the tank with 100% pond water if possible.  If not, then as much as you can.  If you add city water, be sure to add dechlorinator.  Over time, change out the old water with your new water.  You should check the pH, hardness, alkalinity, ammonia, nitrite, nitrate, and oxygen levels in the old and new water for comparison if you can.  By changing the water slowly, it gives them time to get used to it.  Koi grow to a few feet long and, assuming they're just babies, will need a pond soon enough.  I hope you can set one up at your new place.  You'll want to closely monitor the ammonia in your new tank for problems.  You can take some of your pond's biofilter material (if you have it; if not then some gravel or things in the pond will suffice) and put it into your aquarium's filter.

If you don't have access to your new home yet, then just leave the koi there until do.  If you move them to the 90 gallon tank at your old house, then you'll have to move them again when you move.  If this didn't answer your questions, let me know.