The biorb tanks at my lfs are 60l or 15g (are you sure its 8g), In theory you could keep a couple of juvi cons in a 15g until around 2 inches. The main issue with biorb tanks IMO is the water surface area available for gas exchange. I've never owned one of these tanks but from what i understand they have a built in filter. The cons look small in the pics but will soon outgrow the tank, If you dont already have one i would add a air pump to help oxygenate the water but from the pics the fish are not at the surace gasping. It might be better to substitute the earth worms for blood worms if the cons dont eat all the worms remove whats left after a couple of mins to stop it rotting in the tank and affecting the water quality. Speaking of water quality if you dont already do so a partial water change of 25% twice a week will not take long and your fish will appreciate this.
Good luck with the cons Steve