The tiny bug looks like a fungus gnat. They are usually just an annoyance, being harmless to humans. Their larvae eat fungus and decaying organic matter in potting soil. However, when an infestation becomes large enough the larvae start feeding on plant roots. Damage is worse on small plants, and especially seedlings. Here are 3 ways to control them. Fill a shallow container (such as a tuna can) at least 1/4 inch deep with equal parts water and apple cider vinegar, and add a drop of liquid dish soap. Stir gently and set the container on top of the soil where the gnats are congregating. Use yellow sticky traps to catch them. Apply food-grade diatomaceous earth to the top of dry soil. It works best when the plant is watered from the bottom.

If you believe a plant is suffering from root rot, you should remove it from the pot. Cut off any dead or damaged leaves. Pull away the soil and wash the roots. Look for any rotted roots and cut them away. Remove any remaining soil from the pot and wash it with hot water and bleach. Refill the cleaned pot with new potting soil as you replant the damaged plant.

It has become difficult to find a reliable potting soil. I bought a very expensive brand for years, then suddenly plants potted with it refused to grow. Maybe it was a fluke, but I tossed it and went to another brand. Originally, potting soil was primarily peat moss with perlite and vermiculite. Peat is being replaced with coco coir and composted wood waste from sawmills. Sand and soil are rarely used in commercial potting soil mostly because they are too heavy. You should plan on fertilizing every plant in potting soil, but you have to tailor the fertilizer to the plant. That can mean only fertilizing during active growth, or seeking a fertilizer with specific nutrients. The terms natural and organic mean nothing. The USDA does NOT regulate garden products the way it regulates food products.

From the picture the first plant appears mostly to be suffering from some physical damage, tearing or piercing by something. Apart from that the leaves look ok. Is there a drainage hole in the pot?

The Calathea had some tips of leaves which were yellowing and turning brown. The bottom unfortunately does not have a drainange whole. I plan to repot at some point and drill some holes in the pot.

Any pot absolutely must have drainage holes. If there's no hole it's a cache pot and not suitable for planting directly into. You need to repot the Calathea immediately because you can never water it correctly if the pot won't drain. That's very likely why the leaves had yellow tips. Don't wait until 'some point'.

I repotted my Calathea over two weeks ago now and most of the foliage has dried up and died. When I repotted it there was severe root rot, to the point where there was hardly any root mass. Over the course of these two weeks all of the leaves have dried up and died and I had assumed the stress from repotting had killed the plant. Today I finally decided throw it out, however to my surprise when I uprooted it there was a good amount of new root mass growth. I decided to repot it (although maybe this uneeded stress will finally push it over the edge). I have included some photos to illustrate. I'm wondering if it would be best to completely cut back all the foliage in the hopes that the plant will but up new shoots. Any advice is much appreciated!