I'm absolutely baffled

[MaloK]

The good old mercury thermometer, as cheap as can be. remains the reference. I can put all the ones I have in a bucket and they always all concur.

I have a heater that is set to 70 and the tank is maintaining a good 76 and more during seasons.

Depending on how the heater is installed the settings on the thermostat, the resulting temp in the aquarium can vary a lot. A good way to have a good compensation is having a slightly overkill heater in a area with the greatest water movement.

Avoiding to create hot spots is key. The longer it runs when heating and the longer it stays off between uses, the better.

 

[Bamf Comics]

Glad the mystery is solved. I have a double rack of 20highs and the tanks on top run a bit warmer than the ones below almost touching. Heat rises and their closer to the heat supply from the vents overhead. Sunlight changes angles all year long. In my first store I had a row of twelve 10's. Each month as the sun changed angles one tank would turn lime green, the one before and after would go dingy and each month it would progress. I eventually bought a UV to move with the worst tank for a while but had to give up and just move them away from the window. A business that had a goldfish tank I'd service had the same problem 3 months out of the year. cloudy, then green, then cloudy then clear for 9 months or so just to have the problem return the next year. The craziest experience I had one customer fighting the cloudy for months even wrapping the tank in paper. It turns out we finally figured out light was bouncing off a picture frame/mirror in another room then hitting the HOB filter. Spray painted the HOB filter, problem solved finally. Just the last few months I have 3 tubs of close to identical everything under the same 4 ft light getting 3 completely different results. Life is an adventure one mystery at a time.