I cannot really help you with the YouTube part -- this is pure Steam with no involved from the actual game.

Regarding the workshop tanks -- do you get any message if the tank was published successfully or if there was an issue? If you are referring to the image of the workshop submission -- we use the in-game thumbnail (without fish) as initial image for workshop item. You can add any other images to your workshop item using the Steam client. Your tank contains all fish nevertheless if subscribed to by others.

After much song & dance and fancy footwork - I now have a tank in the workshop that works and is linked to a YouTube video of it too! :)
You need to subscribe to the tank you want, even if it's your own tank & restart the program to get the workshop tank to be recognized - as least I had to restart BehindGlass to get it to show right. Then I was able to 'add' the YouTube video from that workshop page with the add images/videos. Now I'm happy. Keep improving BehindGlass cause it can be the best aquarium program out there. Add some more types of Corydorus types - more plant types - more substrates etc. etc. :)

I cannot find any method to publish my aquarium to Steam's Workshop. I checked the forums, & also tried to find some information online, but still no luck on this subject.

There is am icon at the top of each tank in the Tank list, allowing you to publish it. This is option is not available in Simulator mode, but only in Designer mode. It is available for the selected tank (as the buttons for moving the tank left and right at the bottom).

Originally posted by BitBros Games:

There is am icon at the top of each tank in the Tank list, allowing you to publish it. This is option is not available in Simulator mode, but only in Designer mode. It is available for the selected tank (as the buttons for moving the tank left and right at the bottom).

I don't see that on my screen, could you upload a picture showing what yoy mean?

EDIT
Nevermind, I see the workshop is only supported in designer mode and I play in simulator mode because for me, the fun is in creating a set up that is sustainable and would work in the real world. I would love to see the workshop expand to include simulator mode tanks too.