Brown leaves

Ok, I started blooming 4-5 days ago and a lot of leaves near the bottom of my plants are turning brown, dying, and falling off. Should I be concerned?

If they are all falling off the bottom & allthe new growth is healthy & green then It may very well be the plants metabolism working to get all it's sugars into the production of it's buds. She's sending all the sugars to her fruits so she no longer wastes her energy keeping her foliage green.
Natural.

But check your nutes. & don't overwater. Keep stress to a minimum flowering should never be delayed or interfered with.

the leaves shouldn't be dying this early in bloom, it sounds like a nitrogen dficiency. When plants are lacking N the lower leaves will lose pigmentation (turn yellow), and eventually die and fall off. What nutrients are you feeding the plant?

I'm using the prepackaged nutes from Sleathydro. I'll probably invest in something else after this. What would one recommend for a bubbleponics system? But, as Godsend mentioned, the rest of the foliage is nice and healthy and green. It's just at the very base of the plant where underdeveloped leaves have sprouted. I got my PPM meter in but I have to wait to the calibration solution gets here before I can use it. Anyone know how I'm suppose to calibrate and use it doing each plant cycle?