alright i read your thread. It seems you have gotten a lot of mis-information from the store where you bought the fish.
1st read this thread about cycleing- even though you've already stocked your tank you need an understanding of how it's all supposed to work.
http://www.aquariacentral.com/forum...=&threadid=3194
now it sounds like you are testing your water before you add it to the fish tank- this is not correct you are suposed to test the water the fish are swimming around in. So let us know what the results are in your next post.
second i do a 50-60% water change on my tank every week- the more often and the larger the amount of water the better. the bacteria that is needed for the fish to survive is not in the water it is attached to the surfaces in the tank and in the filter media.
the filter should not be changed more than once a month- and you should only change 1 component at a time so that you don't remove all of your good bacteria.
as for you fish- many pet stores will take in fish for store credit if they are healthy. you should try to bring some in to the store keeping only a few and cycle your tank- then go back and get more fish
unltimately i would say these fish would provide a good comunity for your 29: 10 of one kind of tetra, 6-8 corys, and your 3 mollies, you've noticed they have fry- which should also periodically be thined and brought to the pet store.
i suspect when you test your tank water will will find some problem having to do with an unfinished cycle- once the water quality is repaired your fish will probably recover
if you get those readings for me i'll give you a precise plan for what to do.
cheers-Kyle