Add plants recently? Native terristrial plants?
Not sure, this is a new one to me, but if I find a picture of the bugs I think they are I will post it.
EDIT: Until I find it - It should be in the arachnid/aphid family
Water conditions?
Also do an immediate large WC (50%+) get that gravel vac out too!!
Stupid question, but I'll ask - Was you Guppy pregnant?