Dear CR,

Sorry, i may pour cold water on you. Before you start the project, pls do check your condo/apartment deed of agreement, see any rules restraint you building pond at blacony ? Beside, pls do check if your balcony is strong enough to stand the weight.

If you clear for the above point, then i suggest you use custom tank/tub (glass or fiber glass). Don't use cement or lining as leak will cause trouble to your neighbour below. If glass/tub leak, you can repair easily. You can get some idea from bali pond design.

Hope this help.

Regards
eu