Gosh, it doesn't cost much at all - I saw it on the web for less than $4 and then you'd have enough for a long time, too. Might you explain to someone - Dad or someone else who could drive you, that it would cost just as much to get a new fish as to cure this fish, and to have the whole financial investment of your fish tank safe for quite a long time to come? And you could probably pay for it with your own money, if that helps the case ... it makes sense $-wise.

Try that approach, and good luck!