Not really much more to add, I'll echo jyl's coment about using the undergravel filter along with the standard filter; I use this setup and it's great.
If I read you correctly, you lost each fish the day after adding them which may be from adding them too quickly. The method I use is to float the new fish for about 10 min, open the bag and scoop in some water from the aquarium and allow that to sit another 10 min, repeat several times before releasing.
There's an awsome store in my area that has a pretty active forum here:
www.tidelineaquatics.net