There are a number of little pest that fit the description. Several do look like, and more-or-less are flys. Midges, blood-worm flies, most do the real damage in their larval form and can be treated with the same products that works on China Mark Moth larvae.
These products are completely fish safe, not actual toxins or insecticides, but biologic controls. Thuricide and Dipel are both forms of BT. Thuricide is a liquid that is sprayed on the surface of the plants, while Dipel is a powder that is dusted on. Keeping a mosquito dunk near the those plants can also help, as it is yet another form of BT.
If the pest actaully are aphids, however, the BT products won't help.
Mike S.
Spring Hill, FL