All sorts of chironomids including mosquitoes will colonize open water, most are harmless. I used to do it on purpose and net up the larvae to feed my aquarium fish. Those look like fungus gnats to me, but they could be any small midge-like diptera including aquatic species from your water jugs.
Chances are that the amount of chlorine in your water is not going to hurt your plants at all. I add 6 drops of liquid bleach per gallon of solution to my reservoirs every 3-5 days fo keep bacteria and algae growth down. The plants don’t seem to notice.
It is more important to have oxygenated water than zero chlorine, fresh cold water from the tap is nicely oxygenated.
Leave it out for an hour and the chlorine will be 90% oxidized away.
Warm water can hold less oxygen in suspension, so cold water is your friend, within reason, don’t start watering with ice cubes, lol.
I run aquarium airstones in my reservoirs to continuously oxygenate it, and I swear it makes a big difference.
You can probably find municipal water data for your area online, check out how much chlorine is really there. If you need help reading the report, upload it or link to it.
