I couldn't tell you how many people come to me expecting to put 30-40" worth of fish into a 10 gallon aquarium with a bottom end top fin filter... (not to diss top fin.. but a 10g topfin filter is not adequate to even slightly overstock a 10g in my opinion... but then again, I have a rena xp3 and an emperor 400 on a 55g which when my fish grow up will have about 60-65" worth of fish) and then people that think they can keep several fish in a goldfish bowl... (and yes, even wanting to keep tropicals in a goldfish bowl... worst one I was asked was if they could keep a 3" albino tiger oscar in a 2.5g goldfish bowl) its absolutely rediculous. It can be a hassle, but all you can do is inform everyone who you can about the correct setup for each fish they're planning to purchase... and in a few rare cases, I've had to refuse to sell a fish to someone, due to their absolute ignorance and lack of consideration for a living creature. (and my boss backed me up on my decision) I can imagine that my thoughts seemed rather jumbled there... please forgive me, I just got off of work and have been dealing with idiots all day. 
Just got off myself so I know where you're at!! Glad to see some integrity in the industry. We need more, MUCH more! Keep educating. :clap: :clap:
