Why do you keep aquariums?
Answer that, and the focus of your course is created.
I did something similar, but I tied it into the science curriculum, and made my focus adaptation. So we did the easy stuff, setting up and maintaining a tank first. Honestly, there isn't a lot of substance there. It can be taught in a very short time. Looking at fish and how their bodies reflect their origins has teeth. One of my students has become a very respected, leading figure in the commercial side of the hobby. He credits the 'how to make your own biological filter" part for starting his interest.
I have a friend who keeps tanks for plants, with fish secondary, and he taught a free course in a community centre for years. I also worked with teaching kids in crisis about responsibility for other living things using tanks. I know a teacher who has an entire room of tanks, with each cared for by a kid who had previously lived in a freefall environment where no one took charge of anything.
I worked with one child who had an awful home life who made a deal with the school janitor to let her in to the building on holidays, so she could feed the fish. It sounds like nothing, but this was a kid who hid from the school so her home situation would be secret, and getting her in the door of her own volition was a real big step for her.
You never know...
But it is hard to make a longer course on aquariums, because they are fundamentally simple. If you go into breeding fish you can teach biology, if you go into form and function, you can teach evolution, if you go into water testing there's chemistry, and you can focus on ecology, genetics and all kinds of stuff using tanks. Just teaching how to run and aquascape a tank is quick, when you are talking. Typing online, it takes us hours, but face to face, we're efficient.
