Can I add nutes with Bacillus Thuringiensis or afterward?

2 weeks ago I had a fungus gnat infestation. So I let the coco dry out and now that it’s dry should I add BT first then wait a day or 2 before returning to my fert schedule or can I just mix the nutrients with it? Its been 2 weeks since it’s had any nutrients but they seem like they can handle another day or 2. These are mother plants so I’m not really on a big time schedule.

Also another thing how much BT should I use, I’m using this one https://www.amazon.com/gp/product/B004QJ33B4/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1 I’m pretty sure it’s a concentrate. At what rate per gallon should i use. This article doesn’t really say https://www.ilovegrowingmarijuana.com/marijuana-pests-fungus-gnats/.

Something I read a day or 2 ago was that BT doesn’t kill Fungus Gnat Larvae but BTI does. Anyone know if this is true?

Not sure the question but here’s a couple don’t do’s,.

Never add fertilizer to dry soil, never mix different brands without first depleting the salt compounds from the first brand.

The original question was: Should I add BT(which is an insecticide) to my soil first after letting it dry out then wait a few days after washing it out to resume my regular nutrient feeding schedule. Although now I’m reading BT doesn’t the kill the Larvae, instead I might try BTI. Thanks for the tips though. Also why flush the previous brands salts out before starting with another brand?

Plant food makers all use different chemical compounds, some are not compatiblewith others and can bind or antagonize each other, the same way calcium can bind with phosphorus forming calcium phosphate or with sulfur calcium-sulfate(gypsum) it’s the grey stuff that precipitates at the bottom of a reservoir, things bind in still water, bubblers and H2O2 help to greatly slow these natural occurring chemical reactions, without peroxide and or good circulation, the water becomes “swampy smelling”, (hydrogen sulfide).

Different manufactures use different ingredients and at different amounts to achieve their desired antagonistic values to achieve overall chemical compound balance for plant nutrition. Too much of one and not enough of another causes imbalance, in essence, that 's what happens when you switch nutrient makers without flushing.
In soil, it’s also wise not to fertilize when it’s dry, wet it first if it’s dry.