I've been investigating containers. Some like display containers which are the rectangles in my pic but the right size and shape that holds water can be hard to find. The others are 1 gallon oval containers from the dollar store but the plastic isn't as clear.
I used 10 gallon tanks with 4 dividers in each one...slots cut in the tops of dividers. No tank top. I used Eheim Ecco for filtration with intake at one end and outflow at the other. Kept 5 males in each tank. I made the dividers from plexiglass and glued plexiglass strips on the tank sides to slide the dividers down and into and hold them in place.
You can use old beanie baby display cases turned upside down, they're perfectly square at the base and top so you can fit more in less space than with the tupperware that all seem to have rounded corners and lips on the top. They're getting harder to find though.
That's why I mentioned display containers but those exact ones are getting rare and more expensive than other, larger options. I know someone is trying to import a large quantity from other countries to sell but I haven't heard of any progress.