Greetings! I could really use someone's expertise.

I have a 30 gal tank. 3 small fancy goldfish. Water is in excellent condition. I do the regular water changes as recommended. I have a water test kit I use regularly and all is well except for the fish.

I have been struggling with an issue that kills my goldfish one at a time. It's not fin rot and it's not ammonia burn. No one can figure out this problem.

Problem:
Fish tail looks like it's slowly being burned away with a perfect black line all along the edges. It only happens one fish at a time and the others stay healthy until one dies then one other starts showing signs.

The tank is near a window that gets full sun in morning until about 10am and it's in shade. I covered the sunny side in case that might be the issue and I will be moving the tank.

Any suggestions as to what's causing this anomaly?