Glad to hear that you're enthusiastic about the tank
Well, here's my thoughts on what's going on with your situation right now...
Well the problem with the nitrites - your tank is not cycled. Before you can have an "ecosystem" in the tank, bacteria has to develop. Before it develops, there is a nitrite and an ammonia spike. You are in the first half of the cycle, technically, since the nitrites are high. On second thought - do not add any more fish right now (a bit of a change from my previous response. I think you can add the gourami if you want, after the tank finishes cycling).
Petco gave you some misinformation, I would say. The bacteria develops in the gravel, in the filter, and on the walls of the tank - not in the water. Changing the water does not affect it, and can only help your situation. Indeed, it would reduce the nitrite count. I'd hold off on vacuuming the gravel right now, since the tank is still in the early-mid cycle, it'd probably be best to leave the bacteria to develop there. As for the two week schedule - if you stock lightly, you could probably do them every two weeks if you liked it better. With experience you can decide on your own how often you need to do the water changes and how much to change, in order to keep your tank running best.
Hehe, in general as far as trusting lfs advice - listen to them selectively. Sometimes they know what they're doing, sometimes they don't. In any case, verify the advice they give you here, at least here you can get several opinions
Heh, while we're at it, I'd advise you to hold off on the pH regulator. They have a very short-lived effect, and are generally unnecessary. Fish adapt very well to most pH (as long as they're not extreme, not lower than 6 or higher then 8 for example), and anything goes as long as it stays stable and unchanged. Platies are very hardy fish and won't be too bothered.
I'd suggest for you to get some of your own test kits to keep at home, that way there is no need to keep bringing your water to the lfs to get it tested all the time. Just a convenience thing, really. At least you should get pH, ammonia and nitrite. Especially since your tank is cycling, its important to monitor it right now. Also, if you do decide to change the filter you use, make sure to carry over all of the media, the insides of it, into the new one so that the bacteria doesn't have to start growing again and you don't have to start over with the cycle.
Heh, that's all i can think of off the top of my head, really. Hope it helps. Welcome to the Tank, btw
