You're currently keeping your tanks at 73-75F. The Pantanal is a very large river system and the each of its tributaries varies. FYI, A. borelli comes from waters where the temperature ranges from 20-30C (68-86F). It's a very adaptable species. What's interesting is the influence pH and temperature.
The gender determination in this species is highly determined by both the pH and the temperature of the aquarium (correlation is at 79%). At 23C (73.4F), the average % of males are as follows:
pH 4.5: 39.6%
pH 5.5: 37.1%
pH 6.5: 32.8%
As you can see, as the pH rises and the temperature remains the same, the ratio of males to females drops.
Here's the gender determination at 26C (78.8F):
pH 4.5: 68.4
pH 5.5: 53.9
pH 6.5: 68.4
At a higher temperature, the sex ratio balances out at around a pH value of 5.5. Unless you're modifying the water to hit the 5.5 mark, this really isn't achievable.
Gender determination at 29C (84.2F):
pH 4.5: 85.4
pH 5.5: 73.6
pH 6.5: 61.4
At this high temperature, at a pH value of 6.5, the sex ratio is slightly skewed towards males. If you'll noticed, however, as the pH rises, the % of males drop. It actually becomes even more balanced at a higher pH value. At the pH values that I've been keeping my tank at (6.8), the ratio hits close to the 50% range.
Unless you're not worried about breeding these guys, the above information is irrelevant. Otherwise, cranking the temperature up helps.