Yes, it's the Fertilome Ultimate Potting mix. The plants have been really happy in it and have looked really healthy until now.

I'm using a pH probe to test the water and it's not super fancy (one of those $9.99 things you have to constantly replace) and I haven't tested the run off, but the soil is 7.5

Are you saying I'm feeding too frequently? I started with 1/3 tsp per gallon, then 1/2 tsp per gallon, and on the next feeding I was going to increase. Am I only supposed to feed once?

The plants are kind of different heights, and the one that i the oldest and closest to the light (~2 inches) is the one showing the worst signs. I flushed it with vinegar water yesterday and today it looked droopy and the top leaves are really curling under.

I have 3 t12 cfl ballasts with 6 day light bulbs 6,500 kelvin 40w

The leaves that are falling off are from the lower canopy, but there are some nitrogen-deficiency-looking burns on a number of the plants. The newer ones were coming up a lime/yellow color and I treated with epsom salts.

Is there a good way to go about flushing? Should I add lights? Am I feeding too much/frequently?

I'm trying to figure out how to post pictures so I can show you, but for now