hmmmm......have seen these biorbs but very expensive a little suggestion instead if gravel use some play pit sand in your tanks it safe cleaner and looks better and don't damage the bettas fins........i don't do tanks for display as i breed and sell bettas so not the person to ask but i know their are some others on here from UK who keep female tanks and use smaller tanks etc so hope they will post for you......
Unfortunately Spencer is now RIP but you get the idea of the tank. Its around 12 litres
This is a pic of my divided tank
It had beanies floating in there with some jarred boys, but they are gone now so just the 3 males divided. There is a heater and 2 fluval 1 filters in there. Its 37 litres
My only other betta tank is the female community one. 96 litres and yes, there is more than one female in there, although Roxy always comes out and steals the limelight