Lol i love this thread, it was an enjoyable read.
If i may, you have an issue i had as a young man. I did what i was supposed to and tried to make a "sensible" bulletproof arguement just as you have. However just as you have, i constantly set the bar too high. When this thread started different fish would have been enough to satisfy you and the arguement. Your father has given you the original answer you wanted, but now that you have set the bar too high the original is no longer good enough.
You should not let pride hold you back from opportunity. By passing on the offer you are now literally accepting nothing for your effort. $20 dollars of different fish would still be better than nothing and likely save the goldfish you originally wanted to save.
Additionally, it is your parent's home and 50g may be scary to them and you should try and be grateful for opportunities even if they are not exactly what you want.
In any case, chin up, go give your mom and dad a hug and kiss, and tell them you love them. You only get 1 real mom and dad. Good luck!