One way to expand the possibilities of small tanks is to connect tanks together. You can make two kinds of connection.
Some tanks include the connection in the body of the tank. However this can introduce challenges went to comes to cleaning the tank, and add to the cost of the tank.
Any closed tube can be emptied of air using a small tupe--and water pressure will keep air from re-entering. It is still wise to ensure the tube can fully empty without causing the tanks to overflow (in case of accidents). One option for above surface linking is the "Aquabridge" pictured below.
Water links can help stabilise water parameters by creating a larger continuous body of water. They can also offer more activities option for many species. However you may find many fish unwilling to use above surface tubes.