help - worms in substrate: 10mm rectanglar head transparent + 2mm white
My friend found that the substrate in his 50gal tank was full of worms. There are two kinds: one is about 10 mm long and 2-3 hairs thick with brownish/transparent color. Instead of having rounded heads like regular worms, these have flat/rectangular ends. The second kind is white, about 2 mm long and more worm-like. The large worms don't move much but the small/white ones wiggle. He said there were hundreds of them just under the substrate in the front glass where he could see (and who knows how many more there are throughout the whole tank.)
Does anyone know
(1) what these worms are and whether they are harmful to the fishes,
(2) if there is a treatment for this condition, and
(3) where these worms might have come from?
My friend thinks it's from the plants (perhaps the potted ones?) I ordered for him. Could it be from the fishes or the substrate (Eco-complete and flourite)? I feel bad about his condition because he had been running a problem-free bare tank until I got some plants for him. Since then, he got algae, snails and now the worms. So, if you have any idea on how to deal with this worm situation, please let me know. Thanks.
By the way, he only feeds his fishes dried foods (flakes, bloodworms, tubifex etc.) most of the time, and frozen brine shrimps occasionally.