where do gill flukes come from?

Hi.

It looks like my gill fluke problem is back, even after two doses of
Jungle's parasite tablets a few weeks ago. I just bought a canister
filter and UV, so I'm hoping that once I install these (hopefully tomorrow
night) this problem will go away.

Anyway the reason for this post is to ask, where the heck do these flukes
come from, anyway? Is there something I'm doing wrong? I have been
adding plants to my tank, but I bleach dipped them this time purposely to
try and prevent any parasites hitching a ride in on the plants. I do have
a pond snail problem, but my LFS told me to squish them when I see them at
the top of the tank -- this lowers the population and feeds the fish live,
fresh food. Is this perhaps the source of the flukes, crushed snail? I
can't add a big snail carnivore to the tank, because I have a community of
pygmy fish that are all about an inch long. I've been doing regular gravel
vac / water changes, but my work schedule has been keeping me from upping
the frequency of water changes. I'm hoping the extra filtration from the
canister (its a second filter) will partly offset my could-be-better water
change schedule.

So... any input on this topic?

Thanks.

--chris