WOW!! You are getting a 7,500 gallon aquarium; I would like to know, is the tropical fish tank inside or outside the house? How is your monster fish aquarium constructed? What are the dimensions for this huge fish tank?
How to stock your jumbo aquarium with donated orphaned fish?
1) Work with Local Fish Stores. Visit as many pet shops in your area as practical and get to know the fish keeping staff. Give them your card and tell them you have a large aquarium which you are stocking with donated fish which have outgrown their aquarium home. Agree to pick-up the fish in the nearby area, you will learn a lot about how other people take care of their pet fish. I supply the pet shops with interesting pictures of fish which they keep under the counter in a 3 ring binder. I agree to take calls from tropical fish customers with various problems. It takes a short while to build up these relationships; you may already have these contacts in place. I also get a lot of medium large fish donated free by various pet shops when they have grown too large to sell.
2) I take my African Lungfish Darwin to local schools for show and tell in the classroom. You might do the same with something appropriate for school children to see and learn about. I give every child a card with a bright looking fish and something about my orphan fish program. Although I have a web site, see link to Darwin below, this is not a source of donated orphaned fish for me.
http://www.anythingfish.com/images/Ted/PhotoGallery/TedInClassroomWithDarwin.htm
3) I contact individuals doing aquarium maintenance and suggest that they call me when they do a fish change out. Sometimes I have a small aquarium fish which I am willing to swap for a jumbo fish from one of their maintenance aquariums.
4) I belong to the Washington Koi and Water Garden Society and spread the word at the meetings about how I adopt orphaned aquarium fish.
http://www.washingtonkoi.org/
This should get you started. The calls seem to come all at once and then for a while there may be none. During the summer lots of people move and if you are handy you might get not only the fish but the aquarium and all the equipment.
Good Luck