10 gallon

Your filter probably houses a good bit of your beneficial bacteria. If you return all of it your tank will cycle all over again. Is it possible to save the media (sponge, insert, whatever) and only return the mechanical part? If you leave the media in tank water while you get the new filter and then put it into the new filter you'll be in much better shape.

Firstly, to adress the cycling question...yes and no. Your cycle will be set back a little since you will be getting rid of the beneficial bacteria in the filter. BUT most beneficial bacteria live in the gravel, so this shuldn't be a huge problem.

Secondly, if you are buying this filter from Wal-mart, I am going to guess it is a Tetra Whisper filter. You probably just got a defective one. I don't like the Whispers much because the media slot is small, but to each his own. There are tons of thread on which filter is best on this forum if you look around. I reccommend a Bio-Wheel; others will tell you to go with Aquaclear. Both are good options.

Thirdly, in a new tank like yours where the filter doesn't have a lot of bacteria yet, shutting it down probably won't hurt it much. If you have an air pump with a stone or anything like that, keep it running just to improve circulation.

Bottom line, your tank isn't going to do anything without a filter that it wouldn't do with one at this point. If you were asking about algae thinking that it would grow in the time you are at Wal-mart, then don't worry about it. Your filter isn't doing anything to get rid of algae anyway.

*Edit: Just so you know, you should be doing 25% water changes every week regardless of how many fish you have and especially since you have a bit of a crowded tank.

You should get a new filter before you leave for 3 weeks, for sure. If you cant, get a friend or someone to do the exchange. As stated, your filter wont have much to do with algae growth, thats more dependant on water changes, feeding correctly, and amount of light hitting the tank.
As far as the insert you have, I doubt the people at the store will notice if its not there. Keep it. You can pretty much plan on not throwing away any filter media you use, unless its falling apart.

buy the new filter you are going to use, run it for a few days with the old one still running and then return the old filter and everything should have moved itself over to the new filter to help you out a bit....

TheZoo said:

You should get a new filter before you leave for 3 weeks, for sure. ...
As far as the insert you have, I doubt the people at the store will notice if its not there. Keep it.

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Yes, do get the new filter before you go. If this one is shutting down after a few days, you're going to be in for one stinky tank when you get back if you don't.

As to the insert...yes keep it. But tell the people at the store so they can take the other one out of the new filter. You may have to suggest that to them or they will try to deny the return. Heck, keep all the used media you have...that blue pad and the white-ish fibery one. Just let them float in the tank while you do the exchange. Also, please please please dry the filter before you take it back! There is not much worse than stinky filter water all down the front of you for your entire shift (except maybe dead fish slime from the fool who wants his ziplock back after bringing back a dead fish)