I did a small water change on my planted 40 gallon, and noticed a 1/2 inch thread-like worm vigorously wriggling in the water column. I netted it out immediately. It didn't seem to have a head or anything, looked just like a very animated fine piece of thread. Any idea what this is?

It sounds like a nematode. I had these in a fry tank that I had been overfeeding a long time ago. I think they feed on excess food and waste in the tank. I got rid of them by cutting back on the feeding and doing several vacuums of the sand over a few days.

In your case, since you just added your substrate and your tank looks clean, I would think it came in with your plants. Hopefully, you won't see any more of them.

I agree, I had something like that in one of my tanks when I was overfeeding and using a sponge filter. It was a tank with cavebrooders so the fry were microscopic and I was afraid of syphoning out the dirty stuff...in the end the worms were eaten by the parents and not a concern.

It probably did come in with the plants. I'm in the midst of seeing which plants thrive, and which plants rot, so maybe this worm was eating some decaying plant matter. I just siphoned out some dead plant leaves this morning.

Thanks for the help, Aura and Cowchick. :D

Is it important to aggressively get dead leaves siphoned or netted out to avoid problems? (Kinda off topic on thread -- I can re-post elsewhere.)

I think leaving decaying leaves in a tank is like leaving uneaten food in a tank. At the very least, both are going to break down and add to the bio-load of the tank. Too much decaying matter or food, apparently encourages nematodes. I don't think a floating leaf here or there is going to cause any problems. It's more like when a whole plant is unhappy and begins to "melt" that things can get messy.

It is very important if your tank isn't heavily planted to help compensate for the decaying matter. Basically, when organic matter becomes soluble then the first thing to take it up in a low plant tank is algae. So cut your dead leaves...they are unsightly and can cause problems in an aquarium.