Even indoors, personally I don't think that green water is worth the hassle. Sometime, if you have enough eggs, you might try just having clean water and powdered fry food.

Sub tropical climate? So you're not in the UK then.

@@StuartChignell Hi. No, not the UK .....I'm in Asia. But a friend sent me this link about UK retailers only selling medaka for indoor aquariums and to ensure customers understand they cannot be kept in outdoor ponds or barrels ornamentalfish.org/industry-warned-to-ensure-oryzias-species-must-only-be-sold-for-indoor-aquariums/ . Presumably it's not illegal to keep medaka outdoors in the UK ... it appears to be just an advisory warning that government may in future ban the import of live medaka if they become an invasive risk due to being kept outdoors.

Because plants are not necessary and placing them in fry hatching containers increases the risk of introducing mold, disease, etc into the containers. For the first 2 weeks, fry are very fragile .... raising them solely in clean water without plants and other stuff increases the survival rate of the fry.