Well, just make sure the liquid test gets done for right now.
I would look around for other pet more local stores, many times if there's a smaller stores, or a mom/pop local place around they will be more apt to take fish off your hands. Petco and the big chains can be picky about that stuff sometimes.
If all else fails you can maybe have your parents put a craigslist ad up--when I put an ad to rehome my fish I got about 10 emails in a day. Thing is you do
not want to make it so much of a hassle that your parents think its not worth the time effort...they probably don't really understand too well anyways.
Just be calm and collected about it, and explain
rationally to them again why its so important. Don't go 'freaking' out on them or acting like a jabronie about it just so they say "NO" and then you think you're 'off the hook' about it not being 'your fault.' Mistakes get made, we all make them, and that's ok, but keep in mind It's you who overstocked, so you are the one who's gotta do something about it if you want to be
'aquaaaaaa booooyyyyy' in your neighborhood.
(i visualize matt looking like calvin anyways for some reason)