You would be so much better off if you would just listen to what folks are trying to explain to you here. You originally said that your landlord asked you to care for the fish - not take ownership. And then to buy more and dump them on you seems pretty irresponsible IMO. And why would you even consider going in debt to buy a larger tank??? - just doesn't make sense when you have other more important issues going on in your life. If you feel that you want fish, then rehome the Koi to somewhere or someone that has the proper environment for them - and just buy some tropical fish. I had tanks for many years, and there are many fish that would thrive in the size tanks that you have. Try guppies, swordtails, angels, mollies - and there are many more that are inexpensive and easy to care for. I even had a big, ugly goldfish that was named Fred who lived for many years.
Don't feel bad about not being able to provide a good home for the Koi. You obviously didn't know what you were getting in to when you took on this responsibility. If you were able to let Luca go because it was in his best interest - then I'm sure you can let the fish go too.