Loss of fish in established tank

[talpha1]

Hi all. I've kept African cichlids for many (30+) years. About 6 months ago I lost all my fish to a heater malfunction (I woke up to a tank temperature of 99f and 100% loss of all fish). Anyway, I had decided that I would try angelfish, and a planted tank.
Fast forward to today. I'm struggling. I've got great lighting and my plants are doing well, which was a struggle. My tank parameters are really stable and all within range. No ammonia, no nitrites, very low nitrates. My pH is a tad high, usually 7.4-7.0. The water here is quite basic and hard. I do 25% water changes about every 2 weeks, and use pH down, as well as a buffer to level out the pH. I'm getting regular fish loss. The fish seem fine for months after introducing them, then will suddenly disappear (I hunt for the body, and usually, but not always find it), or start acting off and die. Even the plecos don't last long. I have no idea what is going on with this tank. It's a 39 gallon with a fluval 206 filter. I've attached a photo of the most recent sick fish. Other than listless behavior, I do not see any sign of disease. This fish was keeping low in the tank and not moving a lot for about a week prior to beginning to list sideways, upon which I isolated him in a hospital tank.
Any thoughts or ideas. I'm nearly ready to call it quits. I'm so happy with the planted tank, and love my angelfish, but I'm very discouraged.

 

[Sedna]

I've had 0 luck with any blushers I've bought. But to have not only issues with sensitive blushers and then all the other fish as well.. hmm.. That fish specifically, was the eye always that size? It looks kind of large in comparison to body size.
Do you use the liquid drops test kit or do you use paper strips?
When I had issues with PH I switched over to cichlid sand, what do you use for substrate? Is there a possibility that anyone in the tank isn't getting along? Potential treatment for internal parasites? How does their poop look?

** Editing**

Welcome to fishlore by the way. Just saw this is your first post on here.

 

[A201]

Be a good idea not to chemically adjust the PH. Your Angels will likely do well in your unaltered tap water.
Bump the water change schedule up to a weekly 30% - 35%.
Btw. Welcome to Fishlore.

 

[Noroomforshoe]

Sorry about your losses!
You might want to buy an inkbird or other device that monitors the temperature in the tank with very accurate digital technology and actually sounds an alarm if the temperature goes outside of the range that you set.

As for the current losses, I don't recommend adding ph changing chemicals, most fish can adapt to a ph outside of their ideal range. But bouncing Ph is stressful and unhealthy. 7.0 is neutral ph. 7.4 is not hi. If i were you I would quit the ph stuff and see how the fish do. Do you add anything else? you said planted tank, do you add ferts or co2?

 

[talpha1]

I stopped adding fertilizer because I was getting brown algae (diatoms). I also discontinued CO2 because the plants seemed to do good with out it. The pH out of the tap is really high (like 8ish). I use liquid drops to test.
That fish, in particular, has always looked that way. The blush is maybe more pronounced since he started acting off. No aggression in the tank. No signs of parasites, but I could treat it to be sure? Poops look normal. Thanks for the welcome all. Happy to be here.

 

[Sedna]

I stopped adding fertilizer because I was getting brown algae (diatoms). I also discontinued CO2 because the plants seemed to do good with out it. The pH out of the tap is really high (like 8ish). I use liquid drops to test.
That fish, in particular, has always looked that way. The blush is maybe more pronounced since he started acting off. No aggression in the tank. No signs of parasites, but I could treat it to be sure? Poops look normal. Thanks for the welcome all. Happy to be here.

You could treat to err on the side of caution. I can test my water in the morning to show you what my ph reads in my angel tank. Right now it's 1 in the morning for me, and I'm not home, sorry.

 

[Sedna]

Not my usual test strips, my others rolled under my tank lol, but! Here's my ph as reference in that tank (60g) are 2 angels that do very well in that ph.