Listen to the trig guy

Those drain catch trays block drainage so badly that the lowest part of the soil zone becomes waterlogged, anaerobic, and useless to the plant, and may contribute to root rot. So even though the nominal volume is 3 gallons, you may have MUCH less usable soil. Power tools are your friend! 1/2" paddle bit, zing zing and you're done. Then give a nice flush with tap water, and feed immediately with a 1/4 strength grow solution.

Box store pH meters don't give accurate results in water (or soil, often, for that matter!). But tap water is almost ALWAYS safe to use. Unless you are super unlucky and have a really minerally well or municipal water source.