Have you been using a good de chlorinator that can treat chloramines also?
Your tank should be stable unless something in the water is killing off the bacteria.
Poisons like medications kill the smallest forms of life first without causing issue to larger forms of life. So the bacteria are not thriving even though the fish survive.
Are you spraying any aerosols near tank?
Water changes only dilute the bacterial bloom but leave it in same proportions which is why it keeps coming back. Let tank stay idle on water changes. No food for 3-7 days and you can do blackout if you want. You don't have 'green water' so it is not algae related IMO
sterilizer like the 'Green Machine ' is often used for green water but IMO
would likely clear you up within a week either way.