If it's foamy bubbles on the surface not to worry, unless as Squawkbert says they cover too much of the surface. They are not a problem in themselves, but what that would mean is you have an excess of organics breaking down - you sometimes see the same thing along lake shores, turbulence disrupting surface tension (waves - or in the case of your aquarium, the filter) breaks down organics into foam. Basically, you might be overfeeding (too much uneaten fish food or too much fish) or something else organic is breaking down (like dead plant leaves).
Try feeding less and gravel vacuuming more.
When people see the foam on lakeshores, they may think it's detergent, but that's not so.
Try feeding less and gravel vacuuming more.
When people see the foam on lakeshores, they may think it's detergent, but that's not so.