Less long finned black widow tetras. Pointless imbred mutants for the sake of treating fish like ornaments.
Less large plecos.
Less goldfish marketed as tiny fish you can put in bowls. If you're gonna do that sell the much more impressive bettas. Goldfish are carp and grow big. They're practically indoor koi. Personally I'd love to keep some Shubunkin and Calico Fantails in a nice big 60 gallon. That'd be much more awesome than a little stunted half dead thing in a glorified jam jar.
Interestingly, one of my LFS's has a big tank.. I'm assuming it must be at least 50 gallons. It's been a display tank there for a very long time. One day they decided to sell all the fish in it. That was years ago. They're now left with two plecs in there, one is a goldish coloured one, the other one a common plec. They're HUGE. They're both 18" or more, and they just sit there basking under their tree roots and rocks. One is £20 the other £25. Despite their incredibly low price for their size, nobody wants to buy them.
If only people realised they got so huge.
What I don't understand is why LFS don't make a point of selling smaller ones. There are so many varietys of plec, many only reach 2.5". I know about the whole tank seller thing, but it seems so trivial. You'll sell tanks regardless of whether you trick people into buying certain fish. It's amazing to watch. People will come in to that LFS, see the big plecs, gaup at them, notice the price tag, walk on, and then go buy a SMALL plec. What they don't understand is that it's a juvenile version and will end up exactly the same as the one in the big tank! Nobody says, "hey by the way you might want to look at these two, this is how big yours will get".
Stupid stupid stupid stupid.
Also I'd like to see less "finding nemo" fish. That film has probably caused so many botched marine setups.
On a similar note I'd also like to see less 5/8 gallon tanks, with pictures on the box of clownfish swimming around anemones in the tank. Especially if they're not even marine tanks. Because it's just not happening folks, you'll be "finding nemo" floating lifelessly around the rotting hulk of an anemone on some now dead "living rock". If you've got that much money to throw away you might as well get a big tank and learn how to do it properly.