I reseach fish as much as i can before i get them, i even research fish i don't intend on getting.
When i first started out in fishkeeping, i used to go to my local petshops for advice, but this is where i messed up really bad- relying on petshops for advice is always a bad idea IMHO, because of relying on petshops to tell me what i needed to know in the early days of fishkeeping, i ended up with 2 common plecos and a sailfin (still have one common and one sailfin and another common pleco which i adopted of a friend a couple of years ago), and various other fish which were unsuitable for my tanks back then like chinese algae eaters etc.
I certainly don't count asking petshops for advice as proper research, researching your fish is when you sit down at the computer or in a libary and read as much as you can about the fish you intend to buy, going to different sources for your info to get as accurate and up to date advice and info as you can to make up your mind up on. IMHO Visting one site on a particular type of fish on the internet does not count as research just as much as asking a couple of people doesn't count as proper research either IMHO.
Most of us have done impulse buys in the past, but unless you research your fish properly, you can't guarenteee that you have the knowledge to look after your new fish properly not can accomodate it properly in your tanks etc.
If more people researched their fish properly, there would be a heck of a lot less badly stocked tanks (in one way or another) or simply a lot less dead, unhappy or ill pet fish in the world.
Some people say that fishkeeping is easy- it is in some respects, but more than anything, its very easy to make mistakes if you don't do your research properly.
I pretty much know what will and won't work with my tank though, so I'm good. Plus, my LFS puts fish info on signs, like what they eat and how big they get. It's helpful. 
