Mmmmmh, I think you are luckier not to see anything..., perhaps you are not quite so inhabited as I was....lol.
Although many parasites cannot be seen visually.
Then again, I never realised that what I sometimes saw floating in the bowl was a parasite, I used to think it was undigested food!
I bought my $10 zapper from zapperplans.com, they may not be the quickest on the delivery front, but they do give everyone the opportunity to try a zapper at a low cost, unless you make one yourself.