If I have SOIL issues I flush, I’ll start with P.H.’ed R.O. or distilled water. I always check the P.H of the first run off. If the P.H. is good I finish flushing and start adding nutrients after the soil dries “ Nutrients if needed or scheduled ”.
If the P.H. is off I will consider the number and look at what might get locked out in this range then add these nutrients at 1/4 strength after the soil dries.
This is not going to be a quick fix so give it a few days and don’t dwell on the old growth.
PS if I have a gallon pot I will use a gallon of water and stop adding water when I get run off, I note how much water it took by looking at what is left in the jug. this will be the amount I will work with. “ this is assuming soil is ready for water”
for example if I use 1/4 gallon and get run off I will then use another 1/4 gallon to finish the flush. I might go through a few 1/4 gallon sets to change the P.H. and one set to flush extra food.