1 - crappy prefilters that clog
2 - the same theory for humans.. a germ free environment weakens immune systems leaving you more susceptible to getting sick
i dont think there are any cons, but then again, i dont think you really need one. i only have one because i had green water in my planted tank. but in a fish only tank, i just dont see the need for it. ive never ran one in any of my tanks up until now for the green water
1 - crappy prefilters that clog
2 - the same theory for humans.. a germ free environment weakens immune systems leaving you more susceptible to getting sick