Got me - I only started this back at Christmas time!
I can tell you what I've tried over the last month:
"Clarity" - it's a flocci.., flocul..., er, it's some goo that's supposed to make the water clear. Made cloudy water look like thin caramel sauce - didn't work at all. Also read here it's very bad for fish. It went in the trash.
"AmQuel" - gets rid of Ammonia, Nitrite & Nitrate. If you oopsie like I did and add fish way before the tank has cycled (I didn't know any better, honest), you'll need some of this to keep the ammonia/nitrite down.
A good test kit (pH, Ammonia Nitrate, Nitrite, kH, gH). We had the 5-in-1 strips but I think they may have been old since the readings were pretty variable. I went and got the dropper tests instead - seems to be the preferred tests here.
Other than that, I'll say that at first I thought "better fish living through magic chemicals" - have a problem, dump some goo in the tank to fix it. From reading here and on other boards, it seems that chemicals just treat the symptoms - they don't fix the problem. Like painting a wall in your house to cover up water damage; it looks better but you didn't find/fix the leak.
And above all - read, read, read and then read some more! The "sticky" posts here are pretty good. I got a lot of useful info from them.