This is a distinct possibility, and would have been a good thing to stress right away, in the first post, letting those who answer become aware of this potentially dangerous feeding practice.
Disease is usually not related to meal rworms or terrestrial insects, but minnows or goldfish (unless you breed them yourself under high quality conditions) are especially prone to carrying parasites that can easily infect, and commonly kill your fish.
But nitrate, as far as being an integral part of a testing regime should still be considered.
Once a tank is cycled there is little need for regular ammonia or nitrite testing (as long as conditions are stable)
but nitrate and pH are some of the most important indicators of how a system is doing long term.
It's like testing drinking water for E.coli, E. coli are indicators of unsafe conditions for human health.
In the aquarium, nitrate is an similar indicator of unsafe conditions if too high for fish health.
Or having a radically fluctuating pH between water changes can be looked at in the same way.