I think that somewhere in my house is a 1/4" tap, but I'm not sure. Also, remember that JG fittings also come in different sizes! It might be 3/8" I don't even remember.
I used it to tap holes in my PVC Kalkreactor body, and the JG fittings fit in nicely (They fit 1/4" line, so the tap MIGHT be 1/4" though 3/8" sounds right too.) I'll tell you if I find it.
And, don't be too scared, if you drill the (thick) acrylic body of an AquaC and it cracks, or you screw up the threads, just glue acrylic over the hole. Remember the AquaC (and most skimmer) bodies are just boxes, patched holes won't affect performance!
V