Firstly, welcome.
Unfortunately though my news will be neither helpful to your problem or what you want to hear so I do hope others can find a heating solution.
neither of the cats will make it 3 years in a 125 g tank indoors or 125 g pond outdoors, 125 g will cease being big enough to house them anywhere between 9 and 18 months. By which I mean there will be a good chance they will start to show deformities due to cramped living conditions.
I only know of very few instances where these fish remain healthy beyond 18” in confined spaces.
I would suggest that if the population on here do find you a suitable heating solution, such as building liner pond above ground and insulating not only the bottom but the sides too ( which will help against both heat and cold) then you will need to build something larger than your 125g just to get through the 3 years with this pair.