New 75 Gallon tank partially cycled sprung a leak

[Alreadyunderwater]

So my husband and I bought a 75 gallon tank for our family Christmas gift. We researched a bit and mostly listened to the fish store people. Started with 8 tetras. A few weeks went by, it got cloudy and then clear - great we are cycled we thought. We added 3 silver dollars. Then the following week we went a bit nuts and let each kid pick fish. 2 angel fish, 5 fancy guppies and 3 rainbow fish came home. I know now it was all too much too fast. Our ammonia started creeping up and I was doing 15 gallon water changes every few days but then yesterday our tank sprung a leak!!!!
The fish are all currently chilling in a 20 gallon tank. I'm doing twice daily water changes to keep ammonia down.
Tomorrow I get the new 75 gallon tank. What do I do? Start with all fresh water? Try to cycle the tank without the fish? But I don't have another filter. I've stored all the decor in buckets full of water from the leaky aquarium, trying not to kill whatever bacteria we do have.
Please help!!

 

[Essjay]

Fill the tank to check this one is not abut to leak as well, then move everything in there. The filter will have been growing more bacteria while in the 20 gallon (assuming it's in there with the fish) and the extra water volume will mean ammonia and nitrite won't go up quite as fast.

 

[Alreadyunderwater]

Thank you!
Yes the giant 110 tidal filter is on the 20 gallon tank it's a bit comical.
We have a well in Michigan so our water is very hard so we use half RO and half well water to fill the tank. Seems like most of the fish stores in the area do the same. I get the new tank today so hopefully we can get it full and move those fish back home soon.