Impulse. I have never spent the intended amount when going to lfs, I always exceed my sensible limit, and then can't afford to use the oven to make food, and I end up making Easy Mac, while my new fish eat Hikari or Omega.
I mostly impulse buy and then research. And then plan on future tanks. I'm horrible.
But sometimes I get it in my head that I want something, and coincidentally SOMETIMES it works out that I can find it shortly thereafter. In THAT case, then I plan. But mostly impulse. My LFS freaks are fanatics and their enthusiasm is so contagious.
I've started doing allot more research the last 6 mos.. I am doing this with the last few purchases.. And i am much happier, with my fish and self... I still am an addict so impulse is always trying to manipulate me on the simplist of purchases... But i am buying more expensive additions now and would rather not lose them cause i id not know ......
Usually research. Always take quite a long amount of time before deciding to buy which makes it tough buying something I like at the lfs because it gets snapped up pretty quickly.
I always do the research on one or two fish that I want hence I will always get if I run across it, given that I have the space.
others buys are impulse, but only on fish I know about, sometimes I will experiment hoping that the fish will make it in my ray/aro tank. (they usually end up just becoming really expensive feeders!) LOL