If there is a possibility that the old tank was exposed to chemicals, then you should probably not use it for fish. Use it for a plant terrarium instead
. However, if you are dying to use it, then give it a good scrub down with mild detergent, rinse, rinse, rinse, then fill it with water and bleech mixture let it sit for a couple of hrs. Again, rinse, rinse, rinse. Let stand in full sun for few days. Cover the top for a day in the sun, then open it. Give it a good snif for any odor [like oil, chemicals, etc] If it has any odor, don't use it. If it seems ok, give it a try.
Usually, a quarantine tank is set up all the time so that you can quarantine new specimens before it takes up permanent residence in your main tank. You can set up very similiar to how you have your main tank setup or scale it down, if you like. Alternatively, if you don't want a standby tank sitting around the house all the time, you can setup a hospital/qarantine tank just for when you need it [when fish are sick or you want to quarantine a new specimen]. If you go with the permanent quarantine setup, then cycle it with a hardy, inexpensive fish [not a damsel, unless you plan to get rid of it after you cycle]. The reason I say use a hardy, inexpensive [not a damsel] fish to cycle is because: you need it to cycle your tank, if your garage tank is contamonated, using such a fish to test it before bringing home a prized, expensive specimen is wise, and finally, damsels are too aggressive. A damsel alone in a small quarantine tank will become very aggressive and start harrassing any new fish you put in there. You don't want a newly acquired beauty or your favorite old fish that's sick to be tormented to death by a dumb old damsel. Good luck !
[This message has been edited by beth (edited 07-08-2000).]