I've been trying different things with pretty similar results. I've tried with one hole and two. I recently got another tank with a pre installed carto with 2 1/8" holes, much larger than the usual size. I found it flooded when filling. I have now tried my own with just one hole at 1/8" and the flooding is down and seems to perform about the same. It almost seems, within reason, that it doesn't really matter.