cleaning question

I need some advice...

I have a 55g aquarium with one 30-60 Aqua-Tech filter (wal-mart brand). I
have a few tetras, cories, and dainos in there (maybe 10 fish in all). My
questions are... should I add another filter? What brands are recommended
and what size to add to the existing filter. I am planning on more community
fish in the future. How often should I do water changes? And how much? I've
been doing water changes every two weeks with a 25% change. Also, I'm doing
the siphon hose and bucket routine.... which takes me 3 hours. Does the
water changing system that has a hook up to the faucet work well? I was
worried about the temperature and if it can do great damage if it isn't the
same as what is in the tank. And how about the stress coat, does putting it
in the tank water AFTER the new water is added make a big difference?
Thanks, to all responses... I'm really enjoying my aquarium and reading
these newsgroups.

~Kristen