strange uninvited guests

Question
QUESTION: Chris,
I'm at a loss!  Any help would be appreciated!  I've had fresh tanks for about 10 years, but I've never seen something like this before.  When I pull my flossy filter (the sock) there are millions of little brown moving dots.  I will also find them when I add zucchini or romaine.  I cannot see them on my fish and cannot see any apparent distress on my fish, no flashing etc.,. But they might be too small to see on my fish, they are about the size of a granule of salt.  Petco, Petsmart and another fish specialty store in the area all say they have no idea what it is.  Petsmart did call me back after looking at a sample under the microscope and said it looked like a circle with "hairs" around it.  I've been trying to research it...w/out legs it is not a watermite, and waterlice are clear and larger--right?  Ideas?

ANSWER: Hi Dawn;

They are most likely a harmless ciliated protozoan that feeds on bacteria and other microscopic organisms. They are attracted to the veggies because more organisms are growing there for them to eat. It's the same thing that happens in a biology or science class when we put a piece of lettuce in a jar and let it sit for a few days. Tiny little "bugs" grow on the lettuce in the water. Fish breeders do this to have microscopic food for newly hatched baby fish. Your little critters like the filter because other organisms they feed on are consuming the waste in there. These guys are found in ponds and other slow-moving bodies of water and usually referred to as "infusoria". There are many, many types, shapes and colors. If you do an internet search on a few of the words I mentioned you may find it there. If your fish can see them they are probably eating them too. A perfectly natural and healthy food. Just keep the tank clean with water changes and gravel vacuuming weekly so that excess waste doesn't cause other problems that are harmful to your fish.

At Your Service;
Chris Robbins

---------- FOLLOW-UP ----------

QUESTION: Thank you Chris!  I researched as you suggested and it was quite interesting!  The picture of the ciliated protozoan definitely looked like what I was told my "critters" looked like.  It makes sense too that they showed up after I had been out of town for nearly three weeks, I suspicion my well-meaning husband may have over-fed.  My follow up question is if I should just ignore them and proceed as normal with tank maintenance since they are not harmful.  Should I worry either way if they stay or go?  And are they likely to stay?  Thanks!

Answer
Hi Dawn;

Isn't that stuff cool? There are some very interesting creatures out there. Reminds me of biology class in school.

I think you are quite right. Just continue to do regular maintenance and the critters will slowly starve out. They aren't harmful if they remain though. They are doing a job your fish couldn't. If you cut back on food a little bit the fish will eat them too. It's a healthy little snack. They just haven't been motivated to use their natural foraging instincts for a long time, if ever, because they are handed food every day that needs to be eaten right away or it rots. Our fish are so spoiled! ;-)

At Your Service;
Chris Robbins