The surface of my tank is unpleasant. Why?

So lately the surface of my tank has this subtle slick on it, and it's really frothy. Bubbly. Like spit bugs have been around. I started having the bubbles when I got my cories, but it was much less than it is now. The main new thing that came along prior to the surface getting icky was that I got a floating plant the store called "water lettuce" - I don't know it's botanical name, but is a picture of it. It reproduces like tribbles, so I've been thinning it out.
Could the water lettuce be causing my surface slime? Something else? API master test kit testing shows all levels good, and I change the water about once a week.
20 gal
5 cories
6 rosy white finned tetras
6 harlequin rasboras
1 pleco
various plants

Thanks for any input on what might be the cause and fix!

 

What are your test results exactly? And what kind of filtration do you have? At a guess the floating plants are preventing water movement on the surface which creates "stagnant" spots where bacteria can grow.

 

Try buying a circulation pump to get the surface moving. A circulation pump would probably get the job done.

 

For that matter an air stone will work too, probably better for the floating plants since ime they don't like to move around a lot.

 

Oil/protein can build up on the surface and it won't go through the filter because it floats. A surface skimmer can help. I have this one: KollerCraft TOM Aquarium Surface Skimmer

 

This is truly a fantastic forum - the people are so nice, and generous with their time!

Okay, to answer questions. nitrates 0, nitrites 0, ammonia 0, ph about 7.6 (high, I know - it always is. I've been advised thus far that it's better to leave it be and let the fish adjust than to go crazy trying to bring it down - feel free to say otherwise if I need to be hearing different advice)
Filter is an 10-30 gal.

After reading what you all said about surface agitation I realized that the filter was not on it's strongest setting, so I upped that - which is causing a bit more movement. The water lettuce gets pulled to sit right under the cascade of water so I'm sure this is also buffering surface movement.

 

The advice you were given about your pH is correct, it's fine. I've had my bettas in pH's of up to about 8.6 with no ill effects. As long as it's stable it's ok

Are you doing a fish-in cycle or do you have a lot of live plants in your tank? A cycled aquarium normally has at least some nitrates.

As for the plants being pulled into the cascade I wonder if some kind of barrier could be strung across just at the surface where it wouldn't necessarily be seen (maybe a single strip of plastic needlework canvas?) that would stop the floating plants from drifting into the filter.

 

sorry, nitrates are at 5.0, not cycling - been established some time now, just typing too fast.

and yes, barrier sounds like it might work

 

OK, whew!