[Vogata]

Have no doubt that and it concur with own beliefs that this will be the situation in an outdoor system.
We are purely discussing indoor systems, due to the heating issue this is for some of us a real issue. We are talking about from an average of six months (happy days give just four) of indoor (water) confinement. In your setting I assume that most of the water is in natural sun for extended periods before travelling through “the darkness” in a limited time aspect.

We are looking in a Nishikigoi Kichi way to make the most out off our “summer” season (yes inside). There are slim chances of reaching 28-30°C outdoors here even in peaks. Indoors we are now on the way down for the summer season and are as pointed out at 26°C at the moment. Before moving them outdoors we need to be at ~18°C.
We do fantasize that there are more than Nishikigois in a water system that needs/appreciate the sunlight.