Endlers and Guppies are not soft water fish.
Many species that really are soft water fish will have no problems with your water.
Look into Tetras, Rasboras, Barbs and other fish that come from rain forest streams. The GH and KH in these streams is often under 2 degrees, and sometimes almost unreadable, even with the lab quality testing equipment.
pH will be determined by how much organic matter there is in these waters. It can be as low as 5, and certain fish are just fine. It could be anywhere in the 6s and a lot more fish are just fine.
Yes, lets see the test results that include GH, KH and TDS (if you have a meter).
Here are some other things you can experiment with:
Run some tap water in a few jars.
Add baking soda to the jars at different rates. See what it does to the KH and pH, and how long it lasts.
1 teaspoon of baking soda added to 30 gallons of water will raise the KH by 2 German degrees of hardness. In my tanks this raised the pH from 'bottom of the chart low' to 6.2
For smaller amounts of water:
1/8 teaspoon per gallon will raise the KH by 8 degrees. Even this may be too much.
Make 1 gallon with 1/8 teaspoon of baking soda, then fill your jars with a blend of this water + tap water.
1/8 cup hard water + 7/8 cup tap water. (should raise the KH by only 1 degree)
1/4 cup hard water + 3/4 cup tap water. (Should raise the KH by 2 degrees)
Run these tests out several days.
Next, add a handful of whatever substrate you are using to the jars. Now how long does the KH and pH stay in the right range?
Try other materials that add carbonate to the water:
Oyster shell grit, Coral sand, limestone gravel and related materials will also add calcium and magnesium to the water. When you test these also test the GH.
I add these to the filter in nylon bags. They react slower, so will probably not be useful for setting up new water for water changes. They can be added to the substrate (a one-way event: there is no removing it if you add too much!) or added to the filter.
If you find any of these work, here is how to avoid the pH swings:
Prepare the new water for water changes 24 hours ahead. Add whatever minerals you have figured out and circulate the water. For large tanks, or several you might use a garbage can. For smaller tanks a 5 gallon bucket. To circulate the water I use a fountain pump in a garbage can. An air bubbler is fine for a 5 gallon bucket. I use baking soda because it dissolves fast. I also add Seachem Equilibrium for GH because that is what I need. This does not dissolve so easily. When the water is ready it may have the right GH and KH to match the tank, or it may have slightly higher GH and KH to correct the drop that happened through the week. The key is SLIGHT. Do not raise the GH and KH by more than 15% with any one water change.
Add one of these: Oyster shell grit, Coral sand, limestone gravel, to the filter to stabilize the mineral levels between water changes. You could also add carbonates (baking soda or potassium bicarbonate) or GH Booster when you add fertilizer, between water changes. This would sort of even out the drop, making the water more stable through the week.
