I have shot small amounts into aqua soil on the front panel of my tank. However I did not exceed 2 ml per gallon. I use aqua soil and would never think to use a gravel vac.i would think every time you stick it into the substrate your pulverizing it. If it were me I would use my fingers to drag through the material and use the vac as a siphon to get rid of the cloud of dust, but never having it indirect contact.
I missed the size of this tank.
If it were small tank less than say. a 55 gallon I would just replace it.
Best of luck
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my tank is 90-p ADA which is around 48g.