>And the perforation in the Top is to prevent air-pressure build up inside the container which might force fluid into the tubing during injection.
The hole on the lid also allows air in to replace the liquid that's pumped out. Without it the tube won't refil for the next injection. We want some pressure to build so the liquid can rise up the tubing, but we also want the pressure in the tube to be greater than that in the jar.
The only challenging part of building this thing was getting enough flow resistance in the two perforations that the pump could generate the pressure to push the fluid to the tank. My 2.5ml fills up 20" of mini tubing, so I need to supply more than the pressure of 20" of water to make it go up into the tank.
The worst failure mode for this thing would be if the hose comes off the T. That'd pump air straight into the jar, which would push all of the fluid up the tube and into the tank. Mounting it in the cabinet means that you'd have to generate enough pressure to go from jar level to tank level, about 4' in my case. My pump can't produce that pressure even with no air outlet in the jar, so there's no way it can dump the whole jar of ferts into the tank. If the jar were mounted near tank-water level this would be possible (though unlikely the tubing would come off), and if it were above tank level the jar could potentially syphon out if somehow the pump only turned on for a couple seconds and primed the output tube to syphon.
The other likely failure modes involve plugging of one or both of the holes, which stops dosing, or of course failure of the timer or pump, which also stops dosing. You'd notice a deficiency and be able to fix it before it was serious.