I've added a couple of things here. One more thing. Select your fish yourself, say "I want that fish there". Not I want one of those. If they chase the fish around the tank say " No thanks I will Leave it" .
I have been known to say I would like 6 of those or 10 of those and then say "I will catch them it's fine". Especially if you have seen the shop person chasing fish around a tank for the customer before you.
Have they been willing to let you catch the fish? I've never tried that.