Where are the worms? What's been added lately that could have introduced them? You mention flies--could these be the larvae? If so, likely harmless, possibly fish food. Not all fish will go after live foods, but the tetras should.
The bio wheel might need to have the spindle cleaned. The arms that rotate around can get gunk built up, and this will limit flow. As long as the wheel is still turning, you're fine, but keep this in mind if it stops.
The filter behind should be rinsed regularly, to remove the detritus and prevent it from causing problems. I just swish mine around in a bucket of clean water, and run my fingers across the pad to get anything loose.
Good to see you back!