Since the water parameters you describe all look good.
There may be other factors that could cause HLLE.
Are temps above the normal range of 76' F to 82'?
Higher temps often make certain bacteria more virulent.
Is there any stray voltage in the tank, this has been known to create problems.
If you have a scratch or tiny cut on your hands, and it tingles when the hand is immersed in the tank, this is one sign of stray voltage.
What about your tap water conductivity , if high could be problematic.
To find out, go to your City (county) area drinking water quality web site, if the customer base is large enough it should be in the data. Where I lived we tested for hundreds of chemical species, and all results and averages,were posted yearly
You could also call your water supplier to get info.
When I worked for a water supplier as chemist, most fish related water quality troubleshooting questions were routed thru me (even though we had a customer base of over 1 million).
And I was even sent to certain customers houses to test water at times.