Probably due to flouride and/or chlorine your tap water. Try distilled water.

when you water, do you do so where it comes out the drain hole and flushes the soil?

Those pots are way too big for your plants, for one. While you can grow plants in larger pots, you really need a soil that is not so dense for it to work. That soil will be the death of your plants with those pots being so large.

How are you watering your plants? It also looks like fertilizer burn which happens when you water in sips instead of flushing the soil. It could also be root rot.

Root suffocation (overwatering.) A smaller pot won't help as much as a different kind of soil, one that's much more chunky/porous/airy. When the soil is all/mostly tiny particles, there's no air for roots when the soil is moist. Letting it dry out so they can get some air is not that helpful because roots need oxygen & moisture at the same time to function. This puts plants on a cycle of bouncing between 2 inappropriate conditions. Too much moisture while there's no oxygen = rotting/overwatering. Not enough moisture, only air = shriveling/dying.

It just occurred to me that the popular fuss in these fora about overwatering and planting media wouldn't exist, if everyone used plastic nursery pots with multiple holes in the bottom with some holes extending up the sides. My hypothesis is that those common problems are caused by decorative pots that foster overwatering and people are trying to make a silk purse out of a sow's ear. The decorative pot is the sow's ear.

i have a well so there is no fluoride or chlorine in our water.

yes, i usually water until the water comes out the bottom.

i fluff the soil to aerate it.

i water when top 1 inch or so is somewhat dry.

i use decorative pots with holes in the bottom, a plastic pan under them to catch the water.

I have tried smaller pots and gotten the same problem. or had more plants in larger pots. none of my variables or changes seem to make a difference.

There have been some small gnat looking things flying/crawling in and around the soil from time to time. None right now but there were a few months ago.

"Make sure the effluent that exits the drain hole has no pathway back into the soil. "

I disagree. If the puddle of water in the saucer is sufficiently shallow, e.g., 1/8 inch, the large area of water relative to volume that is exposed to air will allow much oxygen to rapidly dissolve into the puddle. Theoretically, a similar oxygenation process occurs in the soil at the bottom of a pot with multiple holes exposed to air. It is only the dissolved oxygen that matters as is obviously true for hydroponics.

Regarding what to do after plant injury by overwatering, the immediate concern is dead roots and anaerobic bacteria. Theoretically, the anaerobic bacteria can be killed by watering with a solution of hydrogen peroxide. I have no experience with this, but you can look up instructions, dilution rate, etc.. And, when hydrogen peroxide decomposes in the soil, pure oxygen is liberated and the spaces in the potting soil that are normally filled with air become filled with pure oxygen.

Another procedure to kill bacteria on roots is to clean the roots and plant in unmilled sphagnum moss, not to be confused with "sphagnum peat moss," Unlike peat moss, sphagnum moss is harvested while alive, and naturally contains antibiotics. Unmilled sphagnum moss is very absorbent but, even when saturated with water, maintains many air pockets. And, as a potting media amendment, unmilled sphagnum moss will introduce more large air pockets than any other amendment. Why isn't sphagnum moss more popular? Because it is expensive on a weight bases. See

http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=%22sphagnum+Moss%22&_sacat=0

"i have a well so there is no fluoride or chlorine in our water."

Have you had it analyzed? Since seeing the movie, The Curse, I am phobic about ground water, see https://en.wikipedia.org/wiki/The_Curse_(1987_film.

All the dissolved solids flushed from the soil are in the effluent - you don't want them making their way back into the soil, else why bother to water correctly in the first place? This is especially true when using fertilizers that have NPK %s significantly different than the ratio at which nutrients are used because nutrient concentrations in the soil solution and effluent quickly become badly skewed and create problems related to EC/TDS and antagonisms.

And now the 'extra holes' are good for gas exchange? That's a switch from improved drainage. If we focus on providing plants with media that supports very little perched water, gas exchange won't be a problem. Gas exchange issues become significantly more limiting as water retention increases and air porosity decreases. Plants sitting in puddles, no matter how many holes in the container, is not conducive to good gas exchange or a well oxygenated rhizosphere.

Something I wrote about using hydrogen peroxide several years ago:

H2O2
has an extra O atom (compared to H2O) in an unstable arrangement.
It's the extra atom that makes it useful in horticultural
applications. Generally, we're not concerned with aerobic forms of
bacteria normally occurring in container media or on roots.

Since
H2O2 is an unstable molecule, it breaks down easily. When it does, a
single O- atom and a molecule of water is released. This O- atom is
extremely reactive and will quickly attach itself to either another
O- atom forming stable O2, or attack the nearest organic molecule.

Reduced
O levels and high temperatures encourage both anaerobic bacteria and
fungi. Many disease causing organisms and spores are killed by O, and
the free O- H2O2 releases is very effective at this. Additionally,
when plants growing in water-retentive media are treated with H2O2 it
will break down and release O into the area around the roots. This
helps stop the O from being depleted in the water filled air soil air
spaces until air can get back into them. High O levels in the rhizosphere can encourage rapid healthy root growth and discourage unwanted
bacteria/fungi.

.

I
know H2O2 comes in several different strengths, the most common of
which are 3% and 35% solutions. Least expensive is the 35% product
which you dilute (to an approximate 3% solution) by mixing 1:11 with
water. I have used the 3% solution at 1-½ to 2 tbsp per gallon as a
cutting dip/soak, and have mixed it into irrigation water for plants
in extremely water retentive soils at up to 3 tbsp per gallon, both
with good results and nothing adverse apparent.

H2O2
in high concentration is a powerful oxidant and quickly oxidizes
almost anything it contacts, so be careful with it if you use it. A
solution that is too strong can destroy any organic molecule it
contacts.

I’ve
seen this chart posted several times as suggested strength solutions
for use in watering plants. You may wish to start at a lower
concentration , such as I’ve used, and experiment.

TO
THIS AMOUNT OF WATER
ADD
THIS AMOUNT OF 3% HYDROGEN PEROXIDE
--OR--
ADD THIS AMOUNT OF 35% HYDROGEN PEROXIDE

1
cup, 3% - add 1-1/2 teaspoons … 35% - 7 to 10 drops

1
quart, 3% - add 2 tablespoons … 35% - 1/2 teaspoon

1
gallon, 3% - add 1/2 cup … 35% - 2 teaspoons

5
gallons, 3% - add 2-1/2 cups … 35% - 3 tablespoons plus 1 teaspoon

10
gallons, 3% - add 5 cups … 35% - 6 tablespoons plus 2 teaspoons

20
gallons, 3% - add 10 cups … 35% - 3/4 cup plus 1 tablespoon plus 1
teaspoon

Al