Back in the day, I kept Heckels in a 55-gallon tank in 100% R/O water. They did fine in the tank. Then I decided to bring the pH down to 5.0 with a drop of acid. The Heckels really thrived in at that lower pH. They swam a lot more and ate more food. Of course, it's hard to maintain that pH. Once the pH crashed and burnt up a school of cories in the tank. The discus were fine. Back then, I just had paper test strips which went down to pH 4.5. So I knew it was at least 4.5, but not whether it was much lower.
This batch is striking for the round shapes. We shouldn't expect wild caught fish to grow to huge sizes. Frankly, these look great to me.
Willie