Aquarium mites?

Question
I have what I can only describe as mites in my tank. When I try and look up aquarium mites on the internet, however, everything I come up with is about white little bugs. These things are brown, roundish, and move very quickly on surfaces or through the water when disturbed. They're everywhere. I don't see any on the fish but they've all been restless recently. I first noticed these mite-like things on just one specific plant. It's a fake plant, though. At first I thought they were tiny snails but they didn't get any bigger.

I pulled the plant out and stuck it in a bowl of water to observe. The largest of these things are the size of a pin head. Once out of the tank I could also see even smaller ones racing around. They're still in my tank, though, and I don't know what I should do. I bought some external parasite medicine and put it in the tank last night but nothing has happened yet. The second dose the box recommended will go in tomorrow evening.Can't get a good picture because they're so small. Does this sound like something you've seen before? I do a 1/4 water change once a week, I don't disturb the gravel because I don't have a gravel filter. I have 1 large pleco, 2 guppies, 6 neon tetras, and a yoyo loach in a 20G tank.

Answer
Hi Jaime,

Most likely, these are harmless Planaria flatworms, but not the dangerous flatworms...moreso the kind which develop from overfeeding and dirty water.

They come in droves once the water reaches a peek of uncleanliness, and survive off of food debris, and breed like wildfire.

If you want to get rid of them, change the water and spotlessly clean your filtration unit. Usually, they show up in tanks not getting monthly water changes.

Make sure to curb feeding to once a day, 1 minute worth of food only.  No food should hit the bottom, ever.

If you need further information, please let me know.

Happy fish-keeping.