What type of bug is it? aphides?

Yup.

and where do they come from?

Eggs.

why does it take so long for them to show up?

Aphids lay eggs late in fall to winter over, therein is why

why don't I see them in the pond, but 5 months after they have been in the
house????

it takes this long for them to hatch, regardless of it being warm inside. I
assume they have a built in dormancy time so they don't wake up too soon.

Submerge the plants for several days, as should get rid of them, and run
your finger around the edges of the tank to smash any that swim over to the
edge during treatment.

I treat all my plants with an insect killer containing Pyrethrins before
bringing them inside, but these plants are not in contact with fish till
the following spring, by which time any chemical exposure (that didn't get
rinsed off) has broken down. My biggest problem has been spider mites over
aphids. ~ jan

See my ponds and filter design:
http://users.owt.com/jjspond/

~Keep 'em Wet!~
Tri-Cities WA Zone 7a
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