PH not stable-help?

Hello, I have noticed that my 75 gallon tank is having an issue with PH staying stable. It seems like it’s about 6.8-7 after a water change then it seems to drop to 6 not long after, and yes I have lost some fish-mostly newer fish. My older fish seem to be doing fine. I’m using API freshwater test kit. After some looking online I read Kh and Gh may affect this but I used tetra test strips which I know are not the most accurate and it said I had soft water but ideal Kh, but don’t know accurate this is. Is there any way to safely adjust my PH so it stays stable. I currently have fish and live plants in my tank. My tank has been cycled for years.
In addition I have read that crushed coral can help. If yes how much do I add I wound prefer to use in my HOB filter if possible. In addition, if that is not the problem will adding crushed coral cause a problem? I have also been battling high nitrates levels for a little bit-doing after changes but it’s a slow process. I left a caretaker in charge of feeding fish when I went away for a weekend and they majorly overfed and I had some live plant death, but that didn’t cause any fish loss, that was a few weeks ago, so I’m not sure if the nitrates are the problem or if it was the PH swing-looking up symptoms it appears the fish were showing more signs of PH shock then nitrate poisoning. My nitrate level currently-which is due for a water change today is about 40ppm with api liquid test kit. I know it’s high and I’m trying to adjust it with water changes as stated above.
In addition I’m also not sure if the nitrates could be causing the ph fluctuation.

 

The tank has been cycled for years. Is the pH issue new or has it always done this?

 

The tank has been cycled for years. Is the pH issue new or has it always done this?

PH issue is new.

PH issue is new.

They only thing I can think of that is difference is we had a new water softner system put in last year. But via my test kit it has neutral ph, 0 ammonia, 0 nirates.

 

the water softener should have a bypass so you can get the regular tap water. or use the outside water hose to refill. that's what i do, we have a water softener.

 

the water softener should have a bypass so you can get the regular tap water. or use the outside water hose to refill. that's what i do, we have a water softener.

Sorry for some reason I just saw this reply. I know this is from a while ago, but is using tap water without a bypass-like the water just from tap causing a problem with my tank. It just happened again and now my nitrates are where they are supposed to be around 5-10ppm and ph is now 6 and I have fish dying.

 

the water from the water softener has had a lot of the hard water minerals removed like calcium, magnesium, and carbonate. this lowers the pH of the water. i've not found any concrete evidence, but some people believe the softened water contains salt, and over time will turn your freshwater tank brackish.

How Water Softeners Work - a Guide to Water Softener operation, maintenance, adjustment
"As hard water which is to be treated flows through the resin or treatment tank containing the salt-coated zeolite resin beads, salt molecules (NaCl) on the bead surface are "swapped" into the water displacing other mineral molecules that we're trying to remove from the water, such as Calcium (Ca or CaCO3) or Magnesium (Mg) that clog up pipes and create other problems.

The Ca or Mg ions stick to the resin beads where they have replaced the NaCL leaving those bad boys stuck onto the resin in the resin tank until the next water softener regeneration cycle. "

Alkalinity and Hardness
"WHY CARE ABOUT ALKALINITY?
Fish and other aquatic life require a pH range of 6.0 to 9.0, and because alkalinity buffers against rapid pH changes, it protects the living organisms who require a specific pH range. Higher alkalinity levels in surface water will buffer acid rain and other acid wastes, preventing pH changes that are harmful to aquatic life. "


Alkalinity and Water
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Why alkalinity is important​

Although you don't often hear about the alkalinity of your favorite lake in the news, alkalinity can be important to the health and welfare of a lake. The ecosystem and organisms that live in the lake evolved in water bodies that didn't change quickly. Before humans came along water bodies were not subjected to chemical spills and acid rain. Likely the pH and aquatic characteristics of a lake did not change much over the short term, which suited the fish in the lake just fine.

In modern times, water bodies can be subjected to sudden inputs of chemicals, such as those contained in acid rain and wastewater, that can cause rapid changes in the acid/base balance of a lake—lowering the pH of the lake water, for instance. A sudden shift in pH is not healthy for the fish and organisms living in the water. Aquatic organisms benefit from stable pH values, and waters with a high alkalinity are better able to maintain a fairly constant pH."