Same two heaters have different temperatures?

[Chunky Monkey]

I use a heater to heat the tank that holds my 4 ACFs and the same exact kind to heat my tank that holds my fish, but the temperature of the frog tank is higher than the fish tank, and if I put my hand in the water I can definitely tell that the frog tank water is much warmer than the water in the fish tank. Why is this? I'm using the exact same heater model and brand.

[mpmistr]

Few variables to consider:

Glass lid used on frog tank? Glass lids will hold in more heat and keep your tank a few degrees above ambient room tempurature.
Same volume of water? A heater will struggle to maintain a higher temperature in a large tank, if your frog tank is smaller than your fish tank it's not going to be as warm.
Placement of tank? Is one closer to a window, does it receive (in)direct sunlight? This could easily affect temperatures on sunny days.
Thermometer on tank? This will certainly tell you how warm the tank is vs. the other. How warm are we talking here? X. Laevis prefers cooler water.

And my last question would be why use a heater in the Summer months?! I live in Illinois which isn't that far from you and well, I keep my house at around 70F, my clawed frog tank is 72F (glass lid) and I do not use a heater, period. Heck I don't even use a heater in the Winter either, I run my heat of course and the tank never goes below 68F. If you're comfortable in your house, so are the frogs.

72F is more or less ideal for clawed frogs and I keep my house rather on the cold side.. so chances are you don't need the heater at all! =)

My suggestion? Remove the heater, honestly you don't need it for this species man they do best at room temperature 68F - 72F is optimal.

[Necromencer]

Following Michael's excellent post:

Is it possible that one of your heaters are faulty? I lost my first pair of African Clawed Frogs which sadly got boiled to death due to a faulty heater. We put so much trust in this exact piece of technology, it is imperative that they are functioning perfectly.

It may be possible that the hotter tank's heater is heating the water more than it should, I don't know...