Look, if you love your fish you will do something about the situation. You should either;
a. Get a bigger tank or dig a pond.
b. Rehome some of the fish to someone who has a large tank or pond with suitable stocking and space for them.
Just give your fish a chance, instead of forcing them to live in a tank too small for them and in temps not suited for them. If you love them enough, you'll give them a chance to thrive, be happy, and live out their long life expectancies as they should do.
Right now you are just treating them like possesions with the way you are keeping them. You may not be able to get a larger tank, but you can at least try rehoming them to someone who can. Every fish deserves to have the chance to lead out a long, happy and healthy life.
Try working on the parent thing though. Explain them your situation, and how at least some of your fish will die at a young age in their current situation if is not changed. How old are you? If you are still in school, work hard at your education and get some good grades to help persuade your parents to allow you to have a larger tank. And anyways, if you are paying for everything in your fishkeeping hobby and doing all the work and looking after and stuff, your parents should be happy that you are taking a responsable approach to looking after your pets- afterall you could be doing a lot worse, like doing drugs and other criminal activities etc.
Be respectful to your parents though- it is their house, and thus their rules. If you can't convince them to allow you to have a larger tank, then the most responsable and caring thing you could do as a fishkeeper is let your fish go to someone who has the right tank for them and can look after them properly etc.