Puzzled by an unexpected benefit

There's been a change to the temperature within the males tank pond system, that I can't fully explain..... And, it's left me with a few degrees C benefit, which I'm not complaining about - but a slight frustration because I can't explain the change.

By way of background, the males tank has 3 other tanks connected to the same filtration system – I use those 3 additional tanks for holding fish we have decided to move on. The males tank system is unheated, and comparing the water temperature against the females tank (when I’m not heating it) - the water temperature in this males tank system is either the same temperature as the females tank, or 1 degree C cooler. That’s the trend that persists throughout the year, and I could reconcile it with the respective positions of the tanks; the with the males pond system located at the end of the polytunnel that gets the most amount of shading from surrounding trees.

Since swapping the tea cup tank which is connected to the males pond system, for a new tank – the males pond system is now 2 degrees C warmer than the females system! On one day last week, the temp difference hit 3 degrees C.

Very strange.

The only thing I can think of that could explain it, is that it’s something to do with the shape and colour of the tank swapped out compared to the tank that replaced it….

A puzzle, for sure; but no complaints about the heat gain.