I know a lot of fish will eat them, gouramis esp.

I always thought they tend to gather in a heavily fed tank. I have had them every now and then, and it was usually a good tank cleaning that eliminated them.

Are your filters up to snuff, not clogged?

Are you a heavy feeder?

As all stated, they are just unsightly, but not an issue, but since most tanks don't have them as a rule (unless they are hidden), I would look for the root cause and address that.

Hope this helps.